These A-list stars, like Brad Pitt and Reese Witherspoon, made memorable appearances on the hit sitcom.

After 10 years and 236 episodes, Friends has been lauded as one of the most successful TV shows in network history. 

Outside of its catchy opening theme song and the cast's irresistible chemistry, Friends is also highly memorable for its slew of celebrity guest actors.

Check out some of the stars who helped make the show's run that much brighter:

Christina Applegate

Courteney Cox as Monica Geller-Bing, Christina Applegate as Amy Green, Matt LeBlanc as Joey Tribbiani

Courteney Cox as Monica Geller-Bing, Christina Applegate as Amy Green and Matt LeBlanc as Joey Tribbiani in the 'Friends' episode "The One With Rachel's Other Sister" (2002).

Playing Rachel's self-absorbed oldest sister, Amy Green, on seasons nine and 10, Christina Applegate made us laugh with her off-the-wall comments on Joey's crazy eating habits and by constantly referring to Rachel and Ross' baby girl (i.e. her niece) as "Ella" (her name was Emma). Despite Ross' protests, Rachel allows Amy to babysit Emma, only to find out that Amy gets the baby's ears pierced without the couple's permission. “Doesn’t it make her nose look smaller!” Amy exclaims, adding insult to injury. Applegate would scoop up an Emmy for the role.

READ MORE: Friends Cast: Where Are They Now?

Reese Witherspoon

Jennifer Aniston as Rachel Green, Reese Witherspoon as Jill Green

Jennifer Aniston as Rachel Green and Reese Witherspoon as Jill Green in the 'Friends' episode "The One with Rachel's Sister" (2000).

As Rachel's youngest sister, Jill Green, Reese Witherspoon demonstrated that being spoiled and self-involved were genetic traits shared with older sis Amy (Applegate). Witherspoon was featured in two episodes of season six where she leaned on Rachel to help her find her independence after daddy cut off her allowance. (Oh, the irony!) While Rachel tried to help Jill find a job, the little sis decided to repay her kindness by going shopping on daddy's dime and... attempting to date Ross.

Brad Pitt

Jennifer Aniston as Rachel Green, Lisa Kudrow as Phoebe Buffay, Brad Pitt as Will Colbert, David Schwimmer as Ross Geller

Jennifer Aniston as Rachel Green, Lisa Kudrow as Phoebe Buffay, Brad Pitt as Will Colbert, David Schwimmer as Ross Geller in the 'Friends' episode "The One With The Rumor" (2001).

Who doesn't love Brad Pitt when he gets to show off his goofy side? In season 8, Pitt played Will Colbert, Ross' former BFF in high school. Thanks to Will, Rachel comes to find out that he and Ross founded an "I Hate Rachel Club" back in the day and spread the rumor that she was born with both male and female parts. Apparently, the rumor was so widespread that even Chandler had heard about "the hermaphrodite cheerleader from Long Island."

A little bit of trivia: Pitt and Aniston were married at that time.

Alec Baldwin

Alec Baldwin as Parker, Lisa Kudrow as Phoebe Buffay

Alec Baldwin as Parker and Lisa Kudrow as Phoebe Buffay in the 'Friends' episode "The One with the Tea Leaves" (2002).

Alec Baldwin playing an insufferable character? No way! Before playing the hilariously conservative and backhanded Jack Donaghy on 30 Rock, Baldwin appeared on two episodes of season 8 as Phoebe's boyfriend Parker, who just gets a kick out of everything and everyone — even showing verbal affection for a picture of Monica's dead dog. He brings his over-enthusiastic attitude to the Gellar's 35th wedding anniversary soiree and basically annoys everyone there, even Phoebe.

John Stamos

John Stamos as Zack, Matthew Perry as Chandler Bing

John Stamos as Zack and Matthew Perry as Chandler Bing in the 'Friends' episode "The One With The Donor" (2003).

Determined to have a baby with Monica, Chandler invites his coworker Zack (played by John Stamos) to dinner on season 9. Chandler's goal? To see if Zack could be the couple's potential sperm donor. At first, Monica thinks Chandler's idea is preposterous, but in the course of, say, 10 seconds, she goes with his plan, and they begin asking Zack — who has no clue what they're up to — a series of questions: "Do you have a family history of mental illness? Diabetes? Gout?" As Zack gets creeped out by their questions, Chandler and Monica further the awkwardness by checking to see if Zack's going bald.

George Clooney and Noah Wyle

Jennifer Aniston as Rachel Green, Courteney Cox as Monica Gellar, George Clooney as Dr. Mitchell and Noah Wyle as Dr. Rosen

Jennifer Aniston as Rachel Green, Courteney Cox as Monica Gellar, George Clooney as Dr. Mitchell and Noah Wyle as Dr. Rosen on the 'Friends' episode "The One with Two Parts: Part 2" (1995).

As part of the show's season 1 lineup, ER stars George Clooney and Noah Wyle play — surprise! — ER docs! As Dr. Mitchell and Dr. Rosen, the two men take a liking to Monica and Rachel, the latter of whom checks into the hospital after spraining her ankle from a fall. The point of conflict starts when Rachel reveals to Monica that she doesn't have insurance and pretends to be Monica to use her health benefits. After the docs ask them on a double date, Rachel and Monica feel compelled to continue the lie and end up getting into a fight, which results in them insulting each other and revealing each other's embarrassing secrets.

Charlie Sheen

Lisa Kudrow as Phoebe Buffay, Charlie Sheen as Ryan

Lisa Kudrow as Phoebe Buffay, Charlie Sheen as Ryan in the 'Friends' episode "The One with the Chicken Pox" (1996).

In season 2 Charlie Sheen plays Ryan, a sailor who only manages to see his former love Phoebe every so often in between his submarine missions. Having planned out a bunch of romantic activities during his two-week stay above sea, Phoebe is devastated when she suddenly gets the chickenpox. Still, Ryan doesn't care and goes in for some smooches and next thing you know, they both have dots of Calamine Lotion all over their faces. Trying hard to avoid scratching their itches, both fall into temptation and scratch each other's bodies with all the sexual innuendo a writer could get away with on a network show.

Julia Roberts

Matthew Perry and actress Julia Roberts hug each other on the set of "Friends."

Matthew Perry and Julia Roberts hug while filming the 'Friends' episode "The One After the Superbowl: Part 2."

Another season 2 megastar that appeared on the show was Julia Roberts who played Susie "Underpants" Moss. As former kiddie classmates to Chandler, Susie recalls how he embarrassed her in front of their elementary school after pulling down her pants. But all of that is in the past, right? As adults, the two go on a date and get hot and heavy. Susie convinces Chandler to take off his clothes and wear her panties in a public bathroom stall, and before he knows what's going on, her revenge is complete after she up and leaves him there in a compromising position.

Sean Penn

Sean Penn as Eric, Lisa Kudrow as Phoebe Buffay

Sean Penn as Eric and Lisa Kudrow as Phoebe Buffay in the 'Friends' episode "The One With The Halloween Party" (2001).

As much as Sean Penn is known for his serious, dramatic roles, his early comedic role as a teenage stoner Jeff Spicoli in Fast Times at Ridgemont High was one for the ages. Fast forward to the early aughts, and Penn rekindled his sense of humor onscreen as a guest star in two episodes of season 8, playing Phoebe's identical twin sister's fiance, Eric. When Phoebe finds out that her sister, Ursula, has been telling Eric a bunch of lies about herself, Phoebe decides that honesty is the best policy. In the end, Eric and Phoebe end up trying to date, but Eric gets too creeped out by her identical looks to Ursula that it doesn't work out.

Winona Ryder

Winona Ryder as Melissa, Jennifer Aniston as Rachel

Winona Ryder as Melissa and Jennifer Aniston as Rachel Green in the 'Friends' episode "The One With Rachel's Big Kiss" (2001).

Rachel kissed a girl and she liked it. In season 7, Winona Ryder played Rachel's college sorority sister Melissa, who comes back into her life and strolls down memory lane. The problem is Melissa claims she doesn't remember one key event that happened between them: The two kissed one drunken night during a college party. When Rachel tells Phoebe about the kiss, Phoebe doesn't believe her. Determined to get Melissa to admit (in front of Phoebe) what Rachel knows to be true, Rachel grabs Melissa and gives her a big smooch in hopes it would jog her memory. It did more than that: Melissa finally confesses she had never forgotten their kiss — and that she's been in love with Rachel ever since.