Be honest: you can't hear “Wicked Game” without thinking about Ross and Rachel getting it on. These are the songs that Friends planted in our brains.
Chris Isaak, "Wicked Game"
As heard in "The One Where Ross and Rachel... You Know." Ross and Rachel have sex to this song in the planetarium.
Source: youtube.com
Grant Lee Buffalo, "In My Room"
As heard in "The One With the List," where Ross gets in trouble for making a list of Rachel's pros and cons.
Source: youtube.com
R.E.M., "Shiny Happy People"
As heard in "The One With the Monkey," in which we meet Marcel. This was also the original theme song.
Source: youtube.com
Smash Mouth, "Every Word Means No"
As heard in "The One Where Ross Moves In" — which has Ross moving in with Chandler and Joey — and later in "The One With the Routine."
Source: youtube.com