Guest starring as ER doctors (get it?), Clooney and Wyle appear in the first season of Friends as cute docs that meet up with Monica and Rachel, after Rachel uses Monica's insurance (and identity) to treat her sprained ankle. Unlike Clooney, Wyle may not be the biggest star in Hollywood, yet he continues his steady TV work in Falling Skies and The Librarians.

Before becoming a huge star in King of Queens, Remini guests on Friends as a single woman giving birth who receives support from Joey while Ross's ex-wife Carol gives birth to Ben.

The Oscar and Golden Globe-winning actress brings sweet revenge to Matthew Perry's Chandler as Susie "Underpants" Moss, seducing then humiliating him like he embarrassed her when they were kids.

The international action star caught Monica's eye but it was Rachel that struck up a conversation and spark with J-C. Drama ensues but luckily nothing involving martial arts.

Monica's sophisticated older boyfriend Dr. Richard Burke charmed viewers not just because he was debonair and handsome, but because he was played the one and only Tom Selleck.

Post-Wall Street and pre-tiger blood, the Anger Management star plays Phoebe's Navy submarine officer who contracts her chicken pox when visiting her on leave.

Ross screws up big-time when he takes Hollywood royalty Rossellini off his list of five celebrities to sleep with and then runs into her at Central Perk. Shouldn't have laminated it, Ross.

Before costarring with Aniston in Along Came Polly, Stiller guest stars as Rachel's date Tommy with serious anger issues and a penchant for screaming at the top of his lungs.

The deadpan comedic actor first guest stars as restaurant owner and Monica's prospective boss, Steve, or as he's better known, "The Stoned Guy." He reappears as Rachel's pitiful blind date in the ninth season.

Famed comedian-actors and longtime friends, Crystal and Williams share a brief cameo as dramatic Central Perk customers Tim and Tomas.

The legendary actor stars as himself in his Friends appearance, in which Joey uses the shower in his dressing room. While he gives Joey much-appreciated acting advice, he also warns him to never, ever use his shower again. Don't mess with The Heston!

Joey and Chandler make the most of their trip to London for Ross's ill-fated wedding, meeting the real Duchess of York and buying a souvenir hat from a street vendor played by Virgin Group billionaire Branson.

Prior to delighting American TV viewers as the irascible Dr. House, Laurie, in his original British accent, proves to be skeptical of Rachel's plan to confess her love to Ross at his wedding. Sitting next to her on the plane ride to London, he asserts, among other things, that Rachel and Ross "were clearly on a break."

The famed actor-director-producer appears as Phoebe's estranged biological father, whose lullaby became the basis for Phoebe's hit song "Smelly Cat."

The A-list actress and producer channels early Rachel as her spoiled younger sister Jill, who visits Rachel to start a new, independent life but ends up romantically pursuing Ross.

Willis scored his second Emmy for his portrayal of Paul Stevens, Ross' (much younger) girlfriend's intimidating father and, subsequently, Rachel's weepy boyfriend.

Academy Award winning actress and ping-pong enthusiast Sarandon mesmerized Joey as Cecilia Monroe, the Days of our Lives star pushed out as Joey takes on her role of Jessica Lockhart. Her actress daughter Eva Amurri guest stars along with her as a soap actress on the receiving end of Jessica' famous slaps.

Having fallen in love with Rachel after a drunken makeout session, Ryder as Melissa reconnects with her former sorority sister years later at Central Perk. Ironically, Ross bumped Isabella Rossellini off his list for Ryder, and he still doesn't get his chance.

Gary Oldman parodies super serious actors playing Richard Crosby, whose drunkenness keeps Joey from officiating Monica and Chandler's wedding on time. Oldman received an Emmy nom for his troubles.

Speaking of super serious actors, Penn takes a more light-hearted turn as Eric, Ursula's fiancé and later Phoebe's boyfriend.

The ultimate Friends guest starring performance comes from then husband to Jennifer Aniston, Pitt in the eighth season Thanksgiving episode, "The One with the Rumor." He plays Ross's childhood friend Will Colbert who co-founded the I Hate Rachel Green Club after Rachel tormented him in high school.

Baldwin sets aside his infamous temper as Parker, Phoebe's unflappably cheerful beau.

Unlike Jill, Amy, played by Christina Applegate is the nasty one of the three Green sisters. While Rachel and Amy often butt heads, at least they can make up by gossiping about Jill.

The Marvel Cinematic Universe's future Ant-Man played Phoebe's boyfriend turned husband, Mike Hannigan.

American acting treasure Goldblum appeared on the show as Leonard Hayes, a director who only takes to Joey's skill once he drinks too much Yoo-Hoo.

DeVito makes an unforgettable cameo as Phoebe's bridal shower stripper, Roy. Despite his "unique" look, the ladies feel badly for him and allow him to finish his "sexy" routine.

Guest starring as himself, Osmond, then host of Pyramid, can't hold a torch to the spectacular failure that is Joey Tribbiani.

Community Dean and Academy Award-winning screenwriter Jim Rash serves as a pivotal series finale guest star, an airplane passenger sitting next to Rachel, echoing Hugh Laurie's unforgettable airplane passenger.