1. Brad Pitt as Will Colbert Tap to play or pause GIF Tap to play or pause GIF NBC Not to be forgotten, Jennifer Anniston's then-husband Brad Pitt guest-starred as one of the founding members of the "I Hate Rachel Green" club. 2. Winona Ryder as Melissa Warbutron Tap to play or pause GIF Tap to play or pause GIF NBC Before she was fighting the Mind Flayer on Stranger Things, Winona Ryder guest-starred as Rachel's sorority sister she made out with one drunken night. 3. Reese Witherspoon as Jill Green Tap to play or pause GIF Tap to play or pause GIF NBC Before she basically took over the world (and our hearts!), Reese played Rachel's sister Jill who tried to sleep with Ross to get back at Rachel. 4. Hugh Laurie as an airplane passenger Tap to play or pause GIF Tap to play or pause GIF NBC Before he won awards for his performance on House, the Stuart Little star gave Rachel a piece of his mind as her seat partner on her way to London to stop Ross' wedding to Emily. 5. Ellen Pompeo as Missy Goldberg Tap to play or pause GIF Tap to play or pause GIF NBC Ellen Pompeo played a fellow classmate who Chandler and Ross competed to date in college. 6. Gabrielle Union as Kristen NBC When Joey and Ross duked it out to win a date with their new neighbor (Union), it didn't go exactly as they planned. 7. John Stamos as Zack NBC We can totally relate to Monica and Chandler in this episode, because honestly, who wouldn't want John Stamos to be their sperm donor? 8. Danny DeVito as Roy the Stripper Tap to play or pause GIF Tap to play or pause GIF NBC When Monica and Rachel needed a last minute stripper for Phoebe's bachelorette party, they were less than enthused when Danny Devito's character showed up. 9. Julia Roberts as Susie Moss NBC Roberts played an old elementary school classmate of Chandler's who he pulled a prank on. She came back into his life and got the ultimate revenge! 10. Bruce Willis as Paul Tap to play or pause GIF Tap to play or pause GIF NBC Bruce Willis appeared on Friends because he lost a bet with his The Whole Nine Yards costar Matthew Perry, and he gave all of his salary from his guest appearance on the show to charity. 11. Jane Lynch as a real estate agent NBC Before she was Sue Sylvester in Glee, Jane Lynch played the real estate agent who showed Janice the house next door to Monica and Chandler's. 12. Jason Alexander as Earl Tap to play or pause GIF Tap to play or pause GIF NBC When Phoebe got a temporary job as a telemarketer, her first call came from a suicidal office manager played by Jason Alexander. 13. Dakota Fanning as Mackenzie NBC Dakota Fanning guest-starred as a young girl who lived in the house Monica and Chandler were buying in the suburbs at the end of the series. 14. And Billy Crystal and Robin Williams as Central Perk customers NBC The comedic duo appeared in Central Perk in the beginning of a season three episode, overstepping their boundaries and cracking one funny joke after another.