Courtesy of friends-tv.org

101 Pilot - What song was playing while Ross and Rachel were each looking out of their windows, during the rainstorm?
The song is Sky Blue and Black by Jackson Browne, from the album I'm Alive.

107 TOW The Blackout - What song where Phoebe, Joey, and Monica singing while Ross was fighting the cat?
Top of the World by The Carpenters.

109 TOW Underdog Gets Away - What songs are featured in this episode?
In the montage showing Joey's V.D. poster all over the city, Don't Stand So Close To Me by The Police was playing. When Ross was singing to his unborn child, he was singing his own version of Hey, Hey, We're the Monkeys, by The Monkeys.

110 The One With The Monkey - What songs were played at the New Year's party?
The song at the beginning of the party is Shiny Happy People, by R.E.M., from the album Out of Time. The song which played while Chandler had photos with Janice and while Phoebe said goodbye to Scientist Guy was Into Your Arms by Lemonheads.
I haven't been able to identify the other song at the party. I you can identify one I've missed, let me know.

111 TOW Mrs. Bing - What song was while Monica and Phoebe took care of Coma Guy?
My Guy by Mary Wells.

112 The One With the Dozen Lasagnes - What TV themes are they humming?
At the beginning of [112] they were singing "The Odd Couple" and the beginning of "I Dream of Jeannie."

115 TOW the Stoned Guy - When Ross has Celia (the dirty talker) over, what music is playing?
Girl, You'll Be a Woman Soon by Neil Diamond.

117 TOW Two Parts, part 2 - What song was playing at the beginning of Phoebe's party (before she arrived)?
What's the Frequency, Kenneth, by R.E.M.

117 The One With Two Parts, part 2 - What song (that went "In a New York Minute...") was playing when Ross and Marcel were in the hospital?
Um, yeah. Okay. That's called New York Minute. It's by Don Henley, on his album The End of the Innocence.

118 The One With All the Poker - What did they whistle in the opening scene?
The theme music from the movie "Bridge on the River Kwai."

118 & 212 The One With All the Poker, and The One After the Superbowl, part 1 - What song did Marcel play in 118, which they also sang to get his attention at the movie set in 212?
The song is "The lion sleeps tonight", also known as Mbube, originally composed by Zulu singer Soloman Linda.

119 The One Where The Monkey Gets Away - What music was playing when Louisa tried to shoot Marcel with a tranquilizer dart?
The theme from the original TV show, "Shaft."

121 TOW the Fake Monica - What song was playing as Marcel was taken away?
The song is You're a Big Boy Now, by John Sebastian.

124 The One Where Rachel Finds Out - What song was playing while Rachel was waiting for Ross at the airport?
Take a Bow, by Madonna, on her album, Bedtime Stories.

201 TOW Ross's New Girlfriend - What music played when Ross and Rachel were talking on the balcony?
Doy! (You don't get a lot of "doy" these days, do you?). That was an instrumental variation on the Friends Theme song, "I'll be there for you."

204 TOW Phoebe's Husband - What music is playing when Ross dances in the street, the morning after sleeping with Julie?
Singing in the Rain, from the movie of the same name, performed by Gene Kelly.

205 TOW Five Steaks and an Eggplant - What Hootie song opened the concert?
I Go Blind. It happens to be featured on the Friends Soundtrack.

207 TOW Ross Finds Out - What music is playing while Monica and Chandler work out?
Macho Man, by The Village People.

208 TOW The List - What song did Ross request on the radio? And what song followed it?
Ross requested With or Without You, by U2, from the album The Joshua Tree.
It was followed by Grant Lee Buffalo's re-make of In My Room (available on the Friends Soundtrack album).

211 TOW The Lesbian Wedding - What songs were used at the wedding & reception?
For the wedding processional, it was a string quartet version of Trumpet Voluntary, by Jeremiah Clarke. Just before and while Ross danced with Susan, it was Strangers in the Night, performed by Frank Sinatra.

213 The One After the Superbowl, part 2 - What song was when Chandler left the restaurant, shielded by the bathroom-stall door?
L'amour est un oiseau rebelle, (Love is like a wild bird), from the opera "Carmen."

213 The One After the Superbowl, part 2 - What song was playing while Ross spent his last day with Marcel?
Looks Like We Made It, performed by Barry Manilow. Music by Richard Kerr, lyrics by Will Jennings.

214 TOW The Prom Video - What song did Ross play on his keyboard in the prom video?
Axel F, by Harold Faltermeye, most common associated with the "Beverly Hill Cop" soundtrack.

215 The One Where Ross & Rachel... You Know - What song was playing when they... you know?
Wicked Game, by Chris Isaak, off the album Heart Shaped World.

217 TOW Eddie Moves In - What song was playing when Chandler and Joey were looking through their windows, missing each other?
All By Myself, by Eric Carmen.

221 TOW The Bullies - What song did Monica and her co-workers at the diner dance to?
YMCA, by The Village People.

223 The One With The Chicken Pox - What song played Ross came into Central Perk wearing the Navy uniform?
That was the Main Title Theme from the movie, "An Officer and A Gentleman," starring Richard Gere and Debra Winger. It's an excellent soundtrack.

224 TOW Barry & Mindy's Wedding - What song played after Rachel sang "Copa Cabana"?
It was Someone To Watch Over Me, commonly performed by Frank Sinatra. You can find it on the album, "Classic Sinatra: His Great Performances 1953-1960."

306 The One With The Flashback - What song started playing when Rachel hit the jukebox in her fantasy about Chandler?
Time of The Season, written by Rod Argent, performed by The Zombies (1969).

308 TOW The Giant Poking Device - What song were Chandler and Phoebe singing at the end?
Endless Love, which is normally a duet with Lionel Richie and Diana Ross.

315 TOW Ross and Rachel Take a Break - What song was played while Ross danced with & kissed the copy girl?
With or Without You, by U2. For the lyrics, see item 208. 318 TOW The Hypnosis Tape - What song played while Monica and Pete were in Italy at the Pizza shop?
That's Amore, performed by Dean Martin.

404 TOW The Ballroom Dancing - To what music did Joey and Treeger dance, on the roof?
Night and Day, performed by Frank Sinatra.

405 The One With Joey's New Girlfriend - What music was playing while Chandler chased Kathy down the street?
The theme from the original TV show, "The Mod Squad."

414 TOW Joey's Dirty Day - What song played at the beginning of the strip club scene?
It's No Good, on Depeche Mode's album "Ultra".

416 TOW the Fake Party - What songs were playing at the party?
One of the songs (that kept repeating "45") is Brimful of Asha, by T. Singh, of the group "Cornershop", from their album When I was Born for the 7th Time. After that song, the next was The Impression That I Get, by The Mighty Mighty Bosstones. This started about the time that Phoebe was talking to Joey about how he was making his sandwich, and then Rachel game in, in her black dress.
After the commercial break, when Phoebe and Joey were sitting in armchairs talking about meat, and then Rachel came in wearing her cheerleader outfit, the music playing was 6th Avenue Heartache, by The Wallflowers.

419 TOW all the Haste - What song does The Singing Man sing each morning?
The song is based on a Flugelhorn Instrumental piece by Chuck Mangione (Man-gee-o-nee) called "Feels So Good." The neighborly "Singing Man" presumably made up the words himself ("Morning's here, sunshine is near..."). Trivia: The writers of the show have mentioned Chuck Mangione in another episode. In The One At The Beach, Phoebe finds a photo on her birth-mother's fridge that she's convinced it her father. However, it turns out to be a photo of Chuck, to whom her birth-mother recently sold a house.

422 TOW the Worst Best Man Ever - What song was playing while Joey remembers all the good times he had with the Duck?
Weekend in New England, performed by Barry Manilow, written by Randy Edelman.

423 TOW Ross's Wedding, part 1 - What song played played while Joey and Chandler first started touring London?
The first song used was London Calling, by The Clash. Several people have asked me what song was used next, while Chandler and Joey toured on the double-decker bus. Unfortunately, that footage was not included in the original US broadcast of this episode, so I don't have that scene. But others have confirmed that it is Roger Miller's England Swings.

504 TOW Phoebe Hates PBS - What music played at the end when Monica and Chandler exited the closet?
Pizzicati, from the ballet Sylvia, by Leo Delibes. (Thank you Jeremy!)

507 TOW Ross Moves In - What song was played at Danny's party?
Every Word Means No by Smash Mouth. This song is on the 2nd Friends soundtrack album, Friends Again. Click the album cover buy:

514 TOW Everyone Finds Out - What song does Chandler put on, to which Phoebe dances seductively?
Makin' a Plan by the group "Semisonic." It is not on any of their albums, but it is on one of the two CD-single releases of their song, Singing in my Sleep. (Semisonic also has one of their other songs on the Friends Again Soundtrack album.)

515 TOW The Girl Who Hits Joey - What song does Ross play at his party, which no one attends?
The Rockafeller Skank, by Fatboy Slim, from the album, You've come a long way, baby.

518 TOW Rachel Smokes - What songs played at Rachel's birthday party?
The first was Never There, from the album Prolonging The Magic, by Cake. The second was Jammin' by Bob Marley.

520 TOW the Ride Along - What music playing as Joey hielded Ross from the "gunshot"?
That's the theme song from the 60's cop show "Ironsides."

522 TOW Joey's Big Break - What song was playing as Joey drove up to the Nevada movie location?
Horse With No Name, by the group "America," on the album America.

601 The One After Vegas - What song did Joey sing to Phoebe after they dropped of the hitchhiker?
Both these episodes used Space Oddity by David Bowie.

606 The One On The Last Night - What song was playing went Chandler tried (but failed) to let Joey win at Foosball?
That was O Fortuna, from the cantata Carmina Burana, by Carl Orff.

606 The One On The Last Night - What song played at the end, when Rachel left, leaving Monica and Chandler alone together in their apartment?
Hmm... you people need to listen better. That was a guitar variation on the Friends Theme Song, "I'll be there for you."

616 TOW It Might Have Been, part 2 - What music was playing during dinner for Monica and Dr. Roger?
Nocturne in E flat, Opus 9, Number 2, by Frederick Chopin.

616 TOW It Might Have Been, part 2 - What music was planing when Rachel was as Joey's apartment (getting drunk)?
That was View From the Other Side, by Duncan Sheik. This song is on the "Friends Again" soundtrack.

617 The One With Unagi - What was the first song on Chandler's mixed tape (Valentine gift to Monica)?
The song is The Way You Look Tonight, often performed by Frank Sinatra. However, I'm told that the version used in that episode was performed by Tony Bennett. The Tony Bennett version is available on many albums, including the soundtrack from "My Best Friend's Wedding."

619 The One With Joey's Fridge - What song played while Ross danced with Elizabeth on MTV?
Delicious, by Semisonic. This song is on the "Friends Again" soundtrack.

620 The One With Mac & C.H.E.E.S.E. - What song played during the Joey / Chandler hug sequence?
"You Really Got a Hold on Me" by Smokey Robinson & the Miracles.

625 The One With The Proposal, part 2 - What music was playing at the end when Chandler and Monica were dancing?
For the actual episode, it was Wonderful Tonight by Eric Clapton. It can be found on the album, Timepieces - The Best of Eric Clapton. It later commercials that contained this scene, the music playing was He is Everything You Want, by Veritcal Horizon.

710 TOW The Holiday Armadillo - What music was playing at the end while they lit the candles?
That was Tradition, from the Broadway musical (and later the movie) "The Fiddler on the Roof."

711 TOW All The Cheesecakes - What music was used at the Geller's cousin's wedding?
There were a couple of slower instrumentals that I didn't recognize. But during the scene where Monica confronted her cousin for not inviting her, there was a more energetic song playing. It is Get Ready, by the group "Rare Earth."

714 The One Where They All Turn Thirty - What usic was playing at Monica's surprise party?
There was more than one song used, and I've only found information on the first one, a swing number called, Big Time Operator, from the album "This Beautiful Life," by Big Bad Voodoo Daddy.

714 The One Where They All Turn Thirty - What music was playing at the end when Ross and Joey finally got Ross's car out of it's parking spot?
That was the theme song from the old TV show "Route 66".

719 The One With Ross and Monica's Cousin - What song played everytime Cassie (Denise Richards) swung her hair around?
I'm Going to Love You Just a Little More, Baby, by Barry White.

722 The One With Chandler's Dad - What music was used in this episode?
In the first Vegas scene, the music was Believe, by Cher, from the album of the same name. Chandler's Dad performed 2 songs: I Feel Pretty, from the musical "West Side Story," and "It's Raining Men," which, I'm told, was originally recorded by The Weathergirls. In the scene where Rachel was driving the Porsche and got pulled over by a cop, it was Ride Wit Me, by Nelly, also featuring City Spud.

800 In NBC's commercials, advertising the episodes of season 8, what song was playing?
Throughout the season, the song most often used in the promos was Only Time, by Enya. It is on the album, "Day Without Rain."

805 The One With Rachel's Date - What song played as Rachel entered the coffee shop and saw Ross with Mona?
It sounds like the intro to Closing Time by Semisonic, which is from the Album "Feeling Strangely Fine."

808 The One With The Stripper - What music played while Monica stripped?
Let Me Blow Ya Mind, by Eve (also featuring Gwen Stefani). It's on Eve's album, "Scorpion."

813 The One Where Chandler Takes a Bath - What music played when Chandler was relaxing in the tub?
Only Time, by Enya. It is on the album, "Day Without Rain."

824 The One Where Rachel Has a Baby, part 2 - What music played after Rachel finally gave birth?
River of Tears, by Eric Clapton.

824 The One Where Rachel Has a Baby, part 2 - What music played when Joey found the ring at the end?
Sign on the Window, by Bob Dylan, from the album "New Morning."

901 The One Where No One Proposes - What music was used in the opening?
The first few moments of episode 901 were a replay of the final scene of episode 824. However, different music was used. A lot of people are asking about the music, but unfortunately, I haven't been able to identify it. If anyone recognized it, let me know. I've got a sound clip of it here if you want to listen.

906 The One With The Male Nanny - What did the nanny play on his recorder (woodwind instrument)?
The music was "Greensleaves," also used for the Christmas carol, "What Child is This."