In the 1990s and 2000s, no show was more popular—or more culturally relevant—than Friends. And, if you were an actor, there was no more coveted role than the guest cameo: Sometimes within a single episode, sometimes spanning across entire seasons, you could be cast as a love interest, a family member, or a total stranger. The network sometimes engaged in "stunt" casting, to bring a famous name onto the show with the idea of boosting ratings. Sometimes one of the actors might bring on a friend. And sometimes the show had a knack for bringing on an actor early in their career before they hit mega-stardom.
If you're familiar with the show, you may remember some of these epic episodes, where someone super cool was hanging out with Monica, Joey, Rachel, Chandler, Phoebe, and Ross (that's Courteney Cox, Matt LeBlanc, Jennifer Aniston, Matthew Perry, Lisa Kudrow, and David Schwimmer, in case you forgot). But we're betting that there are just as many cameos you've forgotten or didn't realize at the time (John Stamos? Gabrielle Union?).