Client Commands
To use a command, just type its name in the console. To access the console press the '~' key above your tab key and below your escape key. If the console doesn't come down, add -console to your command line and console "1" to your config.cfg.

buy
Opens the buy menu to purchase various weapons, equipment, and ammo.

vote & listplayers, listmaps & votemap
To vote for a map type listmaps at the console to see the maps available to vote on, and then type 'votemap x'. Replace x with the corresponding number of the map. Tip: Many servers use admin mod, which makes voting for maps much easier. Simply type 'rockthevote' in normal chat text or wait for a map vote to start. When one begins, you will be notified and you can say (in normal chat) 'vote mapname' (e.g., vote cs_militia). 
Server admins who want to add new maps must also add an entry into mapcycle.txt, this allows players to vote on any new maps. The percentage is defined by mp_mapvoteratio.

To votekick a player type listplayers in the console and then type 'vote x'. Replace x with the number of the player. You need 66%, usually, of your team to vote him off before he is kicked. This percentage can be changed by the server admin using mp_kickpercent.

adjust_crosshair
Changes the color of the crosshair. You can choose from several different colors.

gunsmoke
Typing this command will either turn on or turn off smoke coming from your own weapon. The first time you type it, it will be enabled. The second disabled, and so on. Enabling gunsmoke will lower your frames per second dramatically.

timeleft
Tells you how much time is left on the map in the bottom left hand corner of the HUD.

drawradar & hideradar
These commands show and hide the teammate radar, respectively.

nightvision
This key will toggle night vision goggles on and off when you purchase them.

alias
Not a CS specific command either, using this command in your autoexec.cfg will let you store various commands under one command name. For example: alias [ALIASNAME] "[COMMAND]; [COMMAND]; [COMMAND]; ..." (the three periods denote that the commands can continue, ';...' is not a command to end aliases). Now if you bind [ALIASNAME] to a key or type it in the console, all the commands will execute. Advanced users only. You may want to check out rizzuh's aliases, they display how aliases can be used for your benefit.

menuselect x
Not so much a CS command, this Half-Life command will select an item from the menu (eg buy; menuselect 1; menuselect 4 will buy the p228 pistol).. Used with quickbuy aliases (like rizzuh's aliases).

Client Variables
These are used to change certain aspects, or options, or Counter-Strike.. They can also change performance. To change their values, simply type '[NAME] [VALUE]' at the console.

fastsprites x
0 - regular transparent smoke grenade sprites (best quality)
1 - simple transparent smoke grenade sprites (faster)
2 - VERY simple transparent smoke grenade sprites (fastest)

cl_righthand x
1 (default) to use right hand weapon models, 0 to mirror them to the left hand size.

cl_dynamiccrosshair x
1 (default) to use a crosshair that changes sizes whilst running, jumping, or performing any other activity that affects aim. 0 makes it so the crosshair only changes sizes when firing your weapon, at all other times it is in its smallest form.

cl_corpsestay x
The amount of time (in seconds) dead bodies will stay before sinking into the ground. Dead bodies are cleared at the beginning of each round. Default is 600 seconds (10 minutes), 0 seconds will disable corpses staying around.

cl_minmodels x
1 forces you to play using the minimum model set: leet.mdl, gign.mdl, and vip.mdl. 0 (the default) lets you see all player models. 

max_shells x
Maximum number of shells at one time. Lower values should improve your frames per second.

max_smokepuffs x
Maximum number of smoke puffs at one time. Lower values should improve your frames per second.

hud_centerid x
Set to 1 to center pop text on screen (it will show near your crosshair), 0 to have pop text in the bottom left corner (default).

rate x
This command controls how much information you receive from the server you are connected to. For a 14.4 connection set your rate to 1500, a 33.6k connection should be 3000, a 56k 3500, an ISDN should be 5000, and all connections faster than that should be between 5000 and 9999 (default is 2500). 

Client Info Keys
Use "setinfo [NAME] [VALUE]" to change. For example, setinfo dm 1, will show map briefings. Type setinfo to check the status of all infokey settings.

_cl_autowepswitch x 1 to enable (default) automatically switching to weapons that you pick up and are greater than your current weapon (e.g. a knife will go to pistol, a pistol will go to an SMG or rifle). 0 to disable. _ah x
1 to enable auto-help (the default), 0 to disable. Auto-help gives the player hint messages throughout the game.

_vgui_menus x
Set to 1 to enable VGUI Menus (default), 0 to use the classic, text-only mode.

Server Variables
These variables can only be changed by server operators or users with remote console (rcon). Newest settings on top.

mp_startmoney x
This integer value controls the amount that players start with at the beginning of a new map or when they join a server. 800 is the default and the minimum value, while 16000 is the maximum. 

mp_logdetail x
Use this bitwise svar to control the depth of your logs. Be warned, these options can eat away hard drive space and CPU cycles. Use 0, the default, to log no attacks, 1 to log enemy attacks, 2 to log teammate attacks, and 3 to log both. 

mp_playerid x
This variable controls what players see when they put their crosshair over an enemy, a hostage, or a teammate. At 0 (the default), the player see's all popups with team colors. At 1, the player sees their teammates and hostages, with team colors. At 2, players see no popups. 

mp_forcechasecam x
This is to combat ghosting, or giving enemy locations to your alive teammates while dead. 0 disables such enforcement (default), 1 locks the player's chasecam onto teammates only (players lose the ability to roam), 2 doesn't like the player move or spectate while dead. They stay near their dead corpse. 

mp_fadetoblack x
This is also to combat ghosting, but is more harsh. 0 will have no affect on the game, while 1 will disable chasecam and any sort of death cam at all. The client's screen fades to black instantly after death. One boon of this setting is that corpses stay around for the entire round. Useful for clan matches. 

mp_buytime x
A floating point (decimal) value to determine the buy time in minutes. E.g., '1.25' equals one minute and fifteen seconds of buy time (this is not the pause, but the time where it is legal to purchase stuff). The minimum value is 0.25 and there is no maximum. 

mp_roundtime x
This variable determines the round time, in minutes. After this amount of minutes has passed, a scenario win is awarded (e.g., hostages weren't rescued or the bomb was not planted). This is also a floating point value, but most server operators use integers. The minimum value is 1 minute, the maximum is 9. The default is 5 minutes. 

mp_timelimit x
This is also a floating point value. It's the same as the above option, only it applies to the map and not the round. The default 20 minutes. 

mp_maxrounds x
This, of course, is an integer (whole number). When a two teams finish playing this amount of rounds (before the timelimit or the winlimit is reached) the map ends. 0, or disabled, is the default. 

mp_winlimit x
This, of course, is an integer (whole number). When a team reaches this amount of wins (before the timelimit or the roundlimit is reached), it has won the map. 0, or disabled, is the default. 

allow_spectators x
Setting to 1 will allow spectators, while 0 will disallow them. These are non-playing spectators, not dead people. 

mp_autokick x
This variable is defaulted to 1. Toggles automatic team-killer banning and kicking of idle clients - Setting of 1 = 'on', and 0 = 'off'.

mp_autoteambalance x
This variable is defaulted to 1. Toggles the forcing of clients to join teams to make it balanced - Setting of 1 = 'on', and 0 = 'off'.

mp_c4timer x
This variable is defaulted to 45. Sets the amount of time in between C4 placement and its explosion - ranges between 15 and 90 seconds.

mp_flashlight x
This variable is defaulted to 1. Toggles the use of flashlights by clients - Setting of 1 = 'on', and 0 = 'off'.

mp_footsteps x
This variable is defaulted to 1. Toggles footstep sounds - Setting of 1 = 'on', and 0 = 'off'.

mp_freezetime x
This variable is defaulted to 6. Sets the amount of "freeze" time at the beginning of each round to buy weapons and equipment - Use a setting of '0' to disable.

mp_friendlyfire x
This variable is defaulted to 0. Toggles friendly fire - Setting of 1 = 'on', and 0 = 'off'.

mp_hostagepenalty x
This variable is defaulted to 0, or disabled. Sets the number of hostages a player can kill before they are booted from the server.

mp_limitteams x
This variable is defaulted to 2. Sets the maximum number of players that one team can have more than the other team - Use a setting of '0' to completely disable the team limiting.

mp_logmessages x
This variable is defaulted to 1. Toggles logging of chat messages in the log files - Setting of 1 = 'on', and 0 = 'off'.

mp_mapvoteratio x
This variable is defaulted to 0.6. Set the ratio of players required to vote on the same map before a map will change. The default is 0.6 , which means that 60% of the players on a server must vote on the same map in order for the server to change to that map. The range is 0.0-1.0.

mp_kickpercent x
This variable is defaulted to 0.66. Set the ratio of players on someone's team required to vote to kick the "someone." The default is 0.66, which means that 66% (2/3rds) of the players on his team must vote him off in order for him to be kicked. The range is 0.0-1.0.

mp_tkpunish x
This variable is defaulted to 1. Toggles the forcing of a player to sit out the next round if he has just killed a teammate - Setting of 1 = 'on', and 0 = 'off'.

sv_restart x or sv_restartround x
This variable is defaulted to 0. Sets the amount of time in seconds before resetting the game, including frags, map entities, money, and the amount of time left. Use setting of '0' to disable these restarts. Setting this to 5 would restart the game in five seconds and reset the variable to 0.