We could do an entire gallery on Friends guest stars alone, but the best one, hands down, is Brad Pitt, who appeared on the 2001 Friends episode as an old high school friend of Ross, Monica and Rachel's. Clearly this was amazing because he and Jennifer Aniston were husband and wife at the time.
James Van Der Beek proved to America that he's more than just Dawson when he played the recurring role of Simon on How I Met Your Mother, Robin's high school boyfriend who's Canadian and used to play in a band.
Whenever I picture Will & Grace, I picture Jack doing his version of "If I Could Turn Back Tiiiiime." Jack meets his idol Cher in season 4, and it was the greatest moment of his life. And also ours.
Season 2, episode 2 was the Glee Britney Spears tribute episode, which featured only Britney Spears songs and then Britney herself! She appeared as herself in hallucinations that the glee club kids experienced while under anesthesia and it was awesome.
Christina Ricci played a paramedic on season 2 of Grey's Anatomy, but not just any paramedic. She was the paramedic who had her hand on a bomb inside someone's body as it was about to explode. Basically, this was the most emotional bunch of Grey's episodes ever.
Little tweenagers Mary-Kate and Ashley played "bad girl twins" on 7th Heaven and got Simon Camden in trouble. And then he kissed one of them and his mom walked by. It was awkwardly amazing.
And then in 2012, Beverly Mitchell played a college counselor on Secret Life of the American Teenager, which was created by the same lady who created the 90's show that made her famous, 7th Heaven. Full circle.
Nathan Lane plays Pepper Saltzman, a BFF of Cam and Mitch, on Modern Family. He's the gay BFF everyone wants, honestly, except when it comes to planning your wedding. #disaster
Back when 90210 was a thing, Tori Spelling jumped ship to Saved By The Bell for a few episodes to play Tori, Screech's girlfriend. Her Pippi Longstocking style, in retrospect, was so ahead of the times.
Will Ferrell played Deangelo Vickers on 4 episodes of The Office, the temporary manager of Dunder Mifflin Scranton. When you get Will in a room with Steve Carell, it's just the best.
I don't remember much about Hilary Duff's stint on Gossip Girl, except that her character, Olivia, engaged in a three-way with Dan and Vanessa that got everyone talking.
Jennifer Love Hewitt played Feffy on Boy Meets World, a new girl at John Adams high who made out with Eric. It's just funny because when this episode filmed, she was dating Will Friedle.
Sisters in real life became cousins on screen in 2009, when Zooey Deschanel appeared on that year's Christmas episode of her sister's show, Bones. It was cute.
Katherine played Amber on a 2010 episode of Community and got to makeout with Joel McHale while she was at it. Win/win.
Fine, Stephen Colletti was more than just a guest star on One Tree Hill. He had a major role in the final seasons. But still, we had to include him because it was just fun seeing his real life drama from Laguna Beach happen to him again in the fictional town of Tree Hill.
When Arrested Development returned with 1 new season on Netflix this past summer, the bar for guest stars was set really high. Kristen Wiig and Seth Rogen certainly brought the funny.
It ain't easy to play Adam's first girlfriend post Hannah breakup, but Shiri Appleby nailed it on Girls season 2. Their sex wasn't great so the couple later broke up, but it was inevitable.
Charlie Sheen makes a cameo as himself, albeit a brief one, on The Big Bang Theory when he encounters Raj at a Cheesecake Factory. Pretty sure Raj's life hasn't been the same since.
New Girl's had a plethora of special guests, and while Jamie Lee Curtis gets an honorable mention, Justin Long wins for this one, where he played Jess' boyfriend, Paul. Remember that sex scene they had? You can't not remember that sex scene.
Tyra Banks played Jane, a love interest of Noel's on season 3 of Felicity. Things didn't work out between the two of them but it was a nice fling while it lasted.
James Franco played a doctor at Mindy's practice for the first few episodes of season 2 of The Mindy Project while she was off shacking up in Haiti. We wish Paul and Mindy were the ones shacking up instead, but hey, there's always next season.
Jon Hamm had a three-year stint on 30 Rock, playing Liz Lemon's neighbor and a pediatrician. Three years was not long enough.
Paul Rudd played Leslie's city council opponent on Parks & Recreation. Have you ever seen two funnier people running for office? I certainly have not.
The cartoon versions of Radiohead are beyond amazing. Their one-episode stint on South Park will never go forgotten.
Leslie Mann played a teacher on one episode of Freaks & Geeks, which was produced by Judd Apatow. Judd Apatow is (and was back then) her husband. Get it?
Nikki Reed played one of Ryan's girlfriends on season 3 of The OC alongside Cam Gigandet. Both later on went to star in Twilight! #smallworld
In the second season, Full House set the bar high by getting The Beach Boys to guest star (and perform) as themselves. They were booked as guests on Wake Up San Franciso, but then magically showed up at the Tanner home and hung out with the whole family. Ah, television.
I really missed Alex Mack, until the actress who played her, Larisa Oleynik, joined the cast of Pretty Little Liars as Ezra Fitz's baby mama. Because Rosewood clearly needed more drama.