Friends has had so many famous guest stars on the show that there's too many to count. What's your favorite guest star? Just curious.
Mine most recently would probably be Brad Pitt in the S8 Thanksgiving episode, he was hilarious pretending to hate his real-life wife Jennifer Aniston. Bruce Willis and Reese Witherspoon (both S6) also come to mind. Tom Selleck (S2) also was great. There are many others which I can't think of right now.
-Frank
P.S. Of course, the funniest (and most annoying) not-so-famous guest star has to be woman playing "Oh-my-god" Janice (what's her name?). She pops up when you least expect it, most recently in the very good S8 finale.