1 of 10
''The One With the Rumor'' (Season 8)
Jennifer Aniston's then husband starred in a Thanksgiving episode, playing a very different role: a founder member of the ''I Hate Rachel'' club.
2 of 10
''The One With Rachel's Sister'' (Season 6)
The actress was utterly perfect as Rachel's even-more-spoiled little sister Jill, who shook up her sib when she briefly dated Ross.
3 of 10
Pheobe's best chance at normalcy was marrying geeky pianophile Mike, who became a beloved member of the group during 18 episodes.
4 of 10
''The One With Rachel's Other Sister'' (Season 9)
Rachel's other sister was the despicably witless Amy, who didn't know her niece's name yet still wanted custody if her parents died.
5 of 10
''The One With Joey's New Brain'' (Season 7)
Sarandon played a soap diva (think Susan Lucci) who struck up a romance with Joey after he let it slip her character would be killed.
6 of 10
''The One With Rachel's Big Kiss'' (Season 7)
Nobody believed that Rachel had actually kissed her former sorority sister, played with hilarious closeted enthusiasm by Ryder.
7 of 10
''The One Where Paul's the Man'' (Season 6)
The action star won an Emmy—and got to hook up with Jennifer Aniston—playing the intimidating father of Ross' student girlfriend.
8 of 10
Seasons 2,3, and 6
Magnum P.I. himself romanced Monica as Dr. Richard Burke, a friend of Monica's father and Chandler's biggest competition.
9 of 10
''The One After the Super Bowl: Part 2'' (Season 2)
Roberts got sweet revenge as Susie, who stole Chandler's clothes, stranding him in the restroom.
10 of 10
George Clooney and Noah Wyle
''The One With Two Parts'' (Season 1)
The ER hunks appeared as—this is a stretch—cute doctors who went out with Monica and Rachel.