Have you had a look at this site (link at end) - if you scroll down to the piece about the last show, it mentions that it is a piece of library music, and links to the the previous episode when it was used (by the way, it's not the pilot in which Ross and Rachel first kiss) It seems the music is not available I'm afraid. http://www.friends-tv.org/music.html#207

'Embryonic Journey' by Jefferson Airplane is said to be the guitar music used in the last episode. Have a listen here and see what you think:

http://www.cduniverse.com/productinfo.asp?style=MU
SIC&pid=6079071&cart=187989964

Embryonic Journey was used in the very last scene, not the Ross and Rachel kissing scene

Sorry kags - my mistake ;-)

You don't make many, Cetti!!

My daughter thinks it may be U2 - with or without you

The link in my answer above says that the producers picked a piece of music that sounded like With or Without you becasue they couldn't get the rights to the song in time. The U2 song WAS used in later episodes, most memorably when Ross is dancing with the girl whilst "on a break"

Kags is correct. It's library music, however, it always reminds me of "She Sells Sanctuary" by The Cult.