Here are all the Avatars as of May 18th, that I currently know of. If you have any information about and new unreleased avatars please contact me.
{Thanx to NH for starting out our list! :)

{Click-It Avatars} {Game Avatars} {Pet/Item/NP Avatars} {Other/Retired Avatars}

Click-It Avatars
	Grundo - Faerie Avatar Search for any Faerie Grundo (Click here).
	Pirate! - Krawk Avatar It is located here in the Pirate Academy.
	Pirate! - Aisha Avatar It is located here in the Pirate Academy.
	Pirate! - Shoyru Avatar It is located here in the Pirate Academy.
	Pirate! - Scorchio Avatar It is located here in the Pirate Academy.
	Angelpuss - Angel Avatar Search for the Angelpuss in the Neopets item database.
	Usukicon - Shopkeeper and Usukicon - Usuls Avatars Go to the Usukiland shop, click on the Usukiland Advert link below the shop keeper, and you will automatically get the avatars in your drop down list!
	Grey Faerie Avatar Look up the Grey Faerie neopedia to get this avatar.
	Maraquan Krawk Avatar Look up a Maraquan Krawk (You can grab yourself one here). You'll get a spiffy avatar and random event. Note that the text is a little odd because of that pet's CSS in its lookup.
	Defenders of Neopia - Aisha Avatar Go to this link in the DoN headquarters!
	Defenders of Neopia - Lupe Avatar Go to this link in the DoN headquarters!
	Dr. Death Avatar Go to this link to get the avatar!
	Techo Master Avatar Go to this link for the avatar.
	Plushie Eyrie Avatar Clear your NeoHTML and NeoSignature to a blank, and then set your avatar to "Default". Post a message in the Neopets Message Boards with the word "Squawk" only. Your post will not go through, but you will get a page that says you got the avatar.
	Helpful Zafara Avatar Go to the Help page and type "avatar" in the search toolbar under Sarah the Zafara. Once you reach the avatar help page, keep refreshing until a random event shows up at the bottom of the page.
	Grundo Warehouse Avatar Check out the Grundo Warehouse and where it says "Older 10 character codes", type in A384J for the first input box and 228P1 in the second box. You will be led to a page saying you got the avatar! :)
	Redeemed Avatar Check out the Darigan Chambers page if you fought in the last war and was on Darigans Side, Meridell fighters will NOT get this avatar. Your previous war shield MUST be a Darigan one.
Game Avatars
	Maths Babaa Avatar Get in the Top 50 listing for Maths Nightmare.
	Wheel of Excitement Avatar Spin the Wheel of Excitement and get 10,000 NP as a prize!
	Illusen's Glade Avatar Complete level #20 and higher in Illusen's Glade.
	Jhudora's Cloud Avatar Complete level #20 and higher in Jhudora's Cloud.
	Grundo - Snowthrow Avatar Get in the Top 50 listing for Grundo Snowthrow.
	Capara Avatar Play the first round of Cheat and beat Capara! (If you're already mid-way through the game, I think you have to beat it and then start on the first level again.)
	Better Than You Avatar Beat a contestent at Better Than You!
	Escape from Meridell Avatar Get in the Top 50 Listing for the game Escape from Meridell.
	Freaked Korbat Avatar Get in the Top 50 listing for Korbat's Lab.
	Edna Cackle Avatar Keep doing quests for Edna in her Witch's Tower until you get the avatar. When you will get the avatar is totally random, just keep doing the quests until you eventually nab the avatar.
	Avatar Collector Avatar To claim this avatar, you must get on the Avatar Hi Score List.
	Extreme Potato Counter Avatar Get more than 200 points for Extreme Potato Counter.
	Deadly Dice Avatar Gain this avatar through a random event while playing the Deadly Dice game.
	Plushie Tycoon Avatar Get in the Top 100 listing for Plushie Tycoon.
	Sutek's Tomb Avatar Score more than 2000 points for Sutek's Tomb.
	Bilge Dice Avatar You must score a 24, beating everyone, and a one and four to pass the level in Bilge Dice. Sometimes you may get it, sometimes you may not, you have to just keep trying.
	Lucky Streak Avatar You must win 10 times in a row and get a winning streak in Bilge Dice.
	MAGAX: Destroyer Avatar Get 3000 points or more for MAGAX: Destroyer.
	Chia Bomber Avatar Earn a score of 1300 or more in Chia Bomber to get this avatar.
	Turtum Avatar Get 230 points or more in Ultimate Bullseye.
	Jester Avatar Form a joke that makes the Grumpy Old King laugh. When you get it is entirely random, so just keep trying every day.
	Eliv Thade Avatar Get 1200 points or more in Eliv Thade (anagram game).
	Gadgadsgame Avatar Get 1000 points or more in Gadsgadsgame.
	Weakling NQII Avatar Have Rohane, your main character, lose to a Plains Lupe in Neoquest II to get this avatar. During the entire fight do nothing for any amount of time (5 seconds suggested) and once you land up at your last rest spot you will get this avatar.
	Lil Devil NQII Avatar You must beat the DevilPuss in Neoquest II, Cumulonimbus, located in Chapter 5. (Credit to nicecalculater)
	Dr. Grumps Avatar Score 900 or more in Gourmet Club Bowls to get this avatar. Note that by typing in "shepherd" once the game loads (it will automatically take you to the first round), this will double your score.
	Vexed Avatar Play the game cellblock. The level of tournament you need to be on is COMPLETELY RANDOM!
	Pick Your Own Dung Avatar Play the Pick Your Own game, you must aquire 6 (non-dung) items from the farm (If you do pick up dung, discard it from your basket and continue picking).
	Lenny Plushie Avatar You need to win and solve a Lenny Conundrum match to get this avatar.
	Caption Contest Funny Avatar Have your witty caption picked and win for the caption contest. However, take note you can only get the avatar every 100 caption contests.
	Raiders of Maraqua Avatar Score 800 or more points in the game Raiders Of Maraqua.
	Bunninator NQII Avatar Defeat a Bionic Bunny (an ordinary monster) in Chapter 5 of NeoQuest II to get this avatar.
	Wheel of Monotony Avatar To get this avatar, you must spin the Wheel of Monotony. However, when you get it is random so keep spinning once a day in case you get lucky. (WARNING: One spin costs 100NP, and it can last for hours on end before the wheel stops)
	Tyranu Evavu Avatar To get this avatar, you need to play the game Tyranu Evavu and win at least 15 times in a row. I got this avatar on the 17th win, so I believe it is completly random when you get the av, as long as it is at least 15 wins in a row.
	Invasion of Meridell Avatar To get this avatar, you need to play the game Invasion Of Meridell, Some people got this avatar simply by clicking on the game link, others have got the avatar on Mission 3 Battle 1 and above
Pet/Item/NP Avatars
	Fyora - Faerie Queen Avatar Buy any artifact from her Hidden Tower and you will automatically get it in your Avatar Dropdown List! There is no random event for this.
	Number Six Avatar What you have to do is go to the About Us page and have the same amount of NP as Adam does on hand. (This varies)
	Soup Faerie Avatar Feed your pet at the Soup Kitchen! (This works best at Dying)
	Meowclops Avatar Equip a Meowclops to your active pet, go to the Quick Ref page and refresh till the random event shows up at the bottom of the page.
	Angelpuss Avatar Equip an Angelpuss to your pet (it doesn't matter whether it is your active pet or not), and go to the Quick Ref page and refresh till the random event shows up at the bottom of the page.
	Chocolate! Avatar Buy a Gourmet Food item (R90 - R99) from the Chocolate Factory!
	Space Faerie Avatar Redeem a Rare Item code, get the Space Faerie as a challenger and then defeat her to win this avatar.
	MSPP - The Avatar Buy the Malevolent Sentient Poogle Plushie (TCG). Refresh at your inventory to get the avatar.
	Pant Devil Taunt Avatar Have something that has a "value" (according to it's description) worth over one million NPs. It can be from the Hidden Tower, although Tiki Armour works just as well. Have that object in your inventory, and refresh until you get the random event at the bottom of the page.
	Codestones Avatar Buy all 10 codestones, and then refresh at your inventory to receive a random event.
	Mad About Orange Avatar Have about 10 different orange items in your inventory (i.e. items with the word "orange" in their names), you may need more or less, it's all random, and if you aren't getting it yet then try increasing the amount of orange items in your inventory. You may also have to refresh more than once.
	I'm Smelly Avatar Have about 10 different dung items in your inventory (i.e. items with the word "dung" in their names), you may need more or less, it's all random, and if you aren't getting it yet then try increasing the amount of dung items in your inventory. You may also have to refresh more than once.
	Feed Me Avatar Feed a Skeith any object with the word "skeith" in it (A Blue Skeith Keyring is recommended as it is very cheap).
	Coconut Jubjub Avatar Have a Coconut Jubjub and feed it a Coconut Cocktail.
	Heermeedjet Avatar Defeat the Meerca Brothers to claim *one* of these avatars.
	Meerouladen Avatar Defeat the Meerca Brothers to claim *one* of these avatars.
	Grarrl Warrior Avatar Have a Grarrl pet with you, and then equip a Bony Grarrl Club to it.
	Chokato Avatar Have the Chokato (TCG) card in your inventory and refresh.
	Jetsam Chomp Avatar Feed your Jetsam a Pepito, or any other Aquatic Petpet, to get this avatar.
	Werelupe Avatar You must actually have a Halloween Lupe, then view its lookup to obtain this avatar.
	Evil Fuzzle Avatar To get this avatar, you must play with an Blue Evil Fuzzle toy with your pet and have it scare your pet. This will take you probably, many tries.
	Mystery Island Stamps Avatar Fill your entire Mystery Island Stamp page with all the stamps that go in there.
	Virtupets Stamps Avatar Fill your entire Virtupets Stamp page with all the stamps that go in there.
	Tyrannian Stamps Avatar Fill your entire Tyrannian Stamp page with all the stamps that go in there.
	Haunted Woods Stamps Avatar Fill your entire Haunted Woods Stamp page with all the stamps that go in there.
	Lost Desert Stamps Avatar Fill your entire Lost Desert Stamp page with all the stamps that go in there.
	Black Pteri Avatar Defeat the Black Pteri in the Battledome. You can only gain this challenger through a random event.
	Mootix Avatar You must equip a mootix to any of your petpets, and once it gets attached to the pet's petpet, view your Quick Ref page.
	Drackonack Hungry Avatar Have a Drackonack and Vegan Cheese side by side in your Inventory. Refresh until the Drackonack "eats" the cheese.
	Slorg Avatar Have a Slorg (It does not matter if your Slorg is painted here) attached to your pet for 101 days (or more), view the pet with the Slorg equipped's lookup to get the avatar.
	Mutant Jubjub Avatar Use a JubJub Transmogrification Potion on your pet.
	Whee! Snowbunnies Avatar You must have a Snowbunny equipped to your pet for 1 whole year (365 days) or more.
	Let It Snow! Avatar You must have an Icy Snowflake in your inventory and then vist the Terror Mountain Weather page.
	I Love My Rock Avatar You must have a Sticks N Stones ticket (either from the Ticketbooth or bought from a shop) on a day the band is showing and watch their concert to get this avatar.
	Snorkle Avatar Feed a Snorkle Snout to your pet to get this avatar.
	Alien Aisha Avatar Insert a Nerkmid into the Alien Aisha Vending Machine to win this avatar.
	Buzzin' Avatar You must get a Fountain Faerie quest (This is a random event) and paint your Buzz any color at all through the Fountain Faerie to get this avatar.
	Elephante Surprise Avatar You must feed your Elephante a Bag of Peanuts to get this avatar.
	SDB Pack Rat Avatar Have more than 1000 different items in your Safety Deposit Box to get this avatar.
	Kacheek Swim Avatar Have a Kacheek as your main pet and then visit the beach to get this avatar.
	Kasuki Lu - Heeyah! Avatar Purchase a Kasuki Lu trading card and view its full profile in your inventory to get Kasuki Lu as a challenger. Then defeat him in the battledome to earn this avatar.
	Faboo Avatar Purchase a clothing item from the Clothing Shop that is rarity 80+
	Mazzew Avatar You must have a Mazzew petpet equipped to your pet for at least 250 days. View the pet with the Mazzew equipped's lookup to get the avatar.
	Groovy Chomby Avatar You must have a Chomby and the Fungus Balls ticket (either from the Ticketbooth or bought from a shop) on a day the band is showing and watch their concert to get this avatar.
	Snot Avatar You must defeat Meuka in the battledome.
	Bless you, Shoyru! Avatar Bless a Shoyru with any Bottled Faerie to get this avatar. However, keep in mind there's a very small chance that you (randomly) might not get the avatar, so if it doesn't work the first time, try blessing your Shoyru again.
	Gruslen Avatar Have a Gruslen petpet attached to your pet for 100+ days. The Gruslen can be painted ANY colour for this to work.
	Grey Wocky "Sigh" Avatar You must own a Grey pet and vist its lookup to get this avatar.
	Kougra Fire Paw Avatar Place the Collectable Card Fire Paw into your deck to earn this avatar.
	Ghost Lupe Avatar You must defeat the Ghost Lupe in the battledome.
	Orange Grundo Avatar Have an Orange Grundo and visit your Quick Ref.
Retired/Other Avatars
	Sloth Avatar Gain access to this Secret Avatar through a random event!
	Meridell Knight Avatar You can only get this if you had fought for Meridell in the Meridell War. View your lookup to get the avatar.
	Darigan Darkling Avatar You can only get this if you had fought for Darigan in the Meridell War. View your lookup to get this avatar.
	BFM - Squished Spyders Avatar You can only get this if you had fought in the Battle for Meridell war. If you did, and have a shield in your lookup, view your lookup to get this avatar.
	Free Jhuidah Avatar - Retired Retired means there is no way to get this avatar any more, but while you could you had to create a Togabo Potion in the Cooking Pot and visit the Rock Pool.
	Volcano Reward Avatar- Retired You must have been the first 75 people to solve the Mystery Island Plot correctly.
	I Saved Mystery Island Avatar - Retired You must have been the 10,000th person or lower to solve the Mystery Island Plot correctly to gain this prize.
	TCG Wizard Avatar - Retired You must have guessed the winning contestants correctly in the Neopets TCG Tournament.
	Attack Pea Avatar - Retired You had to get this avatar by visiting the Advent Calendar on December 4th, 2003.
	I *heart* Sloth Avatar - Retired You had to go to Sloth's Mind Control Lab and answer his questions in the following correct order: C, D, B, C, A.
	Jhudora Oh Yeah - Retired You had to go to complete any one quest of Jhudora's to earn this avatar on Jhudora's Day.
	Valentine Chia Avatar - Retired You had to send someone this Neogreeting to get this avatar on Valentine's Day.
	Gadgadsbogen! Avatar - Retired You had to view this Neopedia on Gadgadsbogen Day to get this avatar.
	TCG - Wanna Play? Avatar - Retired You had to guess at least one person out of Captain Canada, Dom Dread, Dragona, and Warren in their correct places in the TCG - Meridell Staff Tournament.
	Illusen Doll Avatar - Retired You needed to complete a quest from Illusen on Illusen Day which was March 17.
	Quiguki April Fools Avatar - Retired You had to go to the news page to get this avatar on April Fools Day, 04/01/2004.
	Easter Bunny Avatar - Retired You had to send this easter neogreeting to someone to get this avatar. It was available only on April 9th to April 11th.