Bruce Willis definitely gave one of the more memorable guest appearances on the show. In season 6, he plays Paul Stevens, the disapproving father of Ross' much younger girlfriend.
In 'The One Where Rosita Dies,' Jason Alexander plays Earl, a depressed office worker to whom Phoebe tries to sell toner. She ends up bringing him back from the brink of taking his own life.
Julia Roberts featured in a single episode in season 2. She played Susie Moss, Chandler's childhood friend who returns to get revenge on him for a prank he played years ago. Late actor Matthew Perry and actress Roberts famously started dating after her appearance.
Winona Ryder put in an appearance during season 7. She played Melissa Warburton, Rachel's college friend with whom she claimed to have shared a drunken kiss.
Towards the end of season 8, Alec Baldwin plays Phoebe's sweet but infuriatingly overenthusiastic boyfriend, Parker.
Helen Baxendale stars in a number of episodes as Ross's second wife, Emily, whom he embarrasses at the altar when he calls her by the wrong name.
Tate Donovan's character Joshua featured in a number of season 4 episodes. He played Rachel's love interest, which was ironic since Donovan had recently broken up with Aniston in real life.
Teri Garr appeared in a total of three 'Friends' episodes. In 'The One At The Beach,' her character, Phoebe Abbott, reveals to main Phoebe that she's her birth mother.
Eddie Cahill was another returning guest star who played a love interest of Rachel's. She initially hired him as her assistant, and then the pair went on to date.
Sources: (Bustle) (Gold Derby)
Jean-Claude Van Damme played himself in the most-watched episode of all time–'The One After the Superbowl' (season 2). The episode originally aired on January 28, 1996.
Michael Rapaport featured in four episodes as Gary, Phoebe's cop boyfriend. At one point he takes the boys on a ride along and they freak out when a car backfires.
Now that we know Monica and Chandler end up together, it's easy to forget that in the beginning there was Richard. The handsome, much older dentist is played by none other than Tom Selleck, who's also set to appear in the reunion special.
Elliott Gould and Christina Pickles featured in a number of episodes across the years. Whenever they did, Jack and Judy simply stole the show.
Richard Branson famously made a cameo appearance in 'The One With Ross' Wedding.' When the gang travel to London for the party, Branson is there selling souvenirs on the street.
Jane Sibbett played the second Carol Willick, ex-wife to Ross and mother to their son, Ben. The original Carol was played by Anita Barone, but she was replaced after one episode.
Phoebe gets a stroke of bad luck in season 2 episode 23 when her Navy beau (Ryan, played by Charlie Sheen) returns to find her riddled with chicken pox.
George Clooney appeared alongside Noah Wyle in season 1 episode 'The One With Two Parts: Part 2.' He played an ER doctor who looked after Rachel after she sprained her ankle.
Brad Pitt appeared in a season 8 episode as Will Colbert, an old friend of Ross' and former president of the I Hate Rachel Club. He was, of course married, to Jennifer Aniston at the time.
Joey and Janine (Elle Macpherson) start out as roommates. They try to have a romantic relationship, but it doesn't work out because she can't get along with Monica and Chandler.
The team at 'Friends' have always loved guest star appearances, and the original series was brimming with them. As we look forward to the guest appearances of new, we thought we'd reminisce a little about those of old.
Check out this gallery to remember some of the best 'Friends' guest stars.
Remembering the best of the 'Friends' guest stars
How many had you forgotten?
TV Characters
The team at 'Friends' have always loved guest star appearances, and the original series was brimming with them. As we look forward to the guest appearances of new, we thought we'd reminisce a little about those of old.
Check out this gallery to remember some of the best 'Friends' guest stars.