Smallville, Dark Angel, and Supernatural.
Best known from Seinfeld, of course. But there's also Pretty Woman, and Hachi: A dog's tale, and Grow Up, Timmy Turner, and some voice work.
He was decent on Heroes, and Once Upon a Time, and iZombie. And I must have seen him in guest roles that I don't remember.
I know him from Night Court; and Dave's World; and a few eps of Cheers; and It; and Spies, Lies & Naked Thighs. And I might have seen him in things I don't remember.
There was Perfect Strangers, several eps of Picket Fences, several eps of Angel, several eps of Lost, an ep of Grimm, and probably various other stuff...
I know him from Big Wave Dave's, and Northern Exposure, and Chicago Hope, a few guest spots on Picket Fences, and China Beach, and an episode of Monk, a couple eps of Commander in Chief, and an episode of Modern Familiy, and an ep of The Carmichael Show.
I know him from Edward Scissorhands; and Escape From Sobibor; and So I Married an Axe Murderer; and The Rocketeer; and Grosse Pointe Blank; and Gattaca; and Get Smart; and Little Miss Sunshine; and a very small role in The Muppets. And he voiced Schmendrick in The Last Unicorn.
The first thing I specifically remember him from is The Chronicle. I've undoubtedly seen him in various guest roles that I don't remember, but these days I mainly know him from Supernatural (starting in season 8), and a recurring role on New Girl (starting at the end of season 2). Also some voice work.
I know him from Arrested Development; and Running Wilde; and a recurring role on 30 Rock; and Series 7: The Contenders; and in 2013 he did a Best Buy commercial. He's also done some voice work. And no doubt I've seen or heard him in some things I don't remember.
Okay, I'm vaguely aware of him as Lou Grant on Mary Tyler Moore, though I never really saw much of that. I liked him in his recurring role on Studio 60 on the Sunset Strip, and there was an episode of The Crazy Ones, and I've probably seen him in some other stuff, though I'm chiefly a fan of his voice work.
Mainly know him from The Addams Family, but he also had recurring roles on Night Court and The Adventures of Brisco County, Jr., and he was in Mr. Boogedy, and Teen Wolf Too. And no doubt I've seen (or heard) him in other stuff. I dunno... he's just got such an appealing loopiness about him, you can't help but like him.
There was Ghostbusters, and Real Genius, and the second season of Defiance, and surely other things that I don't remember.
His best work, I'd say, was Blackadder. There's also Mr. Bean; and Scooby-Doo; and Johnny English; and a minor role in Four Weddings and a Funeral; and voice work in The Lion King.
Mainly know him as Odo from Star Trek: Deep Space Nine. There's also Benson; and Boston Legal; and The Wild Wild West Revisited and More Wild Wild West; and a couple eps of 1600 Penn. And some voice work.
I know him from the Ghostbusters movies; and I've seen a bit of his work on Saturday Night Live; and there's The Blues Brothers; and The Great Outdoors; and My Stepmother Is An Alien; and My Girl; and probably some other stuff. Also voice work in The Gift of Winter and Legends of Oz: Dorothy's Return.
Mainly I think of him for his voice work on The Simpsons and other things, but there's also live-action work. I liked him as Nat the dog-walker in Mad About You. And as the Blue Raja in Mystery Men. There was a small role in Now and Then. And I must have seen some of his other work, I'm not sure....
I know him from Footloose; and X-Men: First Class; and... maybe other things, but damn, it's crazy that that's all I can say for sure.
I know him from Danger Theatre; and The Drew Carey Show; and an episode of Psych; and American Housewife. And various voice work.
Most familiar, of course, from Quanum Leap, Enterprise, and also for awhile on Murphy Brown. And a small role in American Beauty. And a few episodes in the final season of Desperate Housewives.
I know him from Beetlejuice, and 30 Rock, and The Aviator, and Blue Jasmine, and occasional appearances on Saturday Night Live, and some other things that I don't remember as well as I should. There have also been commericals for Capital One (though it seems like those ads are possibly done as his character from 30 Rock, I'm not sure), and Hulu, and probably other things. And he voiced North in Rise of the Guardians.
Quite liked his work on Doctor Who and the spin-off series Torchwood. He was also decent on Arrow. I guess there were a few episodes of Desperate Housewives.
I always thought he was pretty cool as Rufio in Hook, but mainly I know him for his voice work.
Mainly know him from The Pretender, but there's also Kindred: The Embraced, an episode of The Dead Zone, an episode of Castle, and Vampires: The Turning, and probably some other stuff.
I mainly know him from Forever Knight, but there's also The Outside Chance of Maximilian Glick, and Darkman III, and I've probably seen him in some other stuff...
I know him from American Pie, and Loser, and Anything Else, and Prozac Nation, and Mad Love, and an episode of Deadbeat.
I know him from I Still Know What You Did Last Summer; and Nacho Libre; and School of Rock; and The Muppets; and a two part episode of Ghost Ghirls (of which I guess he's a producer); and I saw part of Year One (it was playing at a Thanksgiving gathering I was at in 2010). And there's some voice work, most notably the Kung Fu Panda movies. And I have a CD by his band Tenacious D. And I've surely seen him in other things I've forgotten, and there's plenty of other stuff I want to see.
The first thing I specifically remember him from is Airwolf; but I also know him from The Poseidon Adventure; and The Black Hole; and Escape from New York; and The Single Guy; and Gattaca; and RED; and he voiced Mermaid Man on SpongeBob SquarePants.
I suppose I know him best for Everybody Loves Raymond. But there's also Young Frankenstein, and The Dream Team, and one of the most memorable episodes of The X-Files, and a small role in Species II, and probably some other stuff.
I know him from Wild Wild West; and Swing Kids; and Mary Shelley's Frankenstein. And he voiced Miguel in The Road to El Dorado. And I must've seen him in some Shakespearean stuff, though I don't really remember. Definitely want to see more of his work though.
I know him from Homicide: Life on the Street (though I barely remember the show); and Thief; and The Tuskegee Airmen; and Last Resort; and Brooklyn Nine-Nine. I've probably seen him in some things I don't remember, and he's done some voice work, such as Darkseid in Superman/Batman: Apocalypse.
I know him from The Wizard, and Harts of the West, and Maximum Bob, and Voyage of the Unicorn, and one of the most memorable episodes of Desperate Housewives, and P.T. Barnum, and Charlotte's Web, and The Good German, and probably some other stuff.
I know him from Tron, and The Big Lebowski, and True Grit, and voice work in The Last Unicorn. And undoubtedly some roles that I've forgotten.
Ferris Bueller's Day Off, The Cable Guy, Election, Inspector Gadget, Finding Amanda, Then She Found Me, voice work in The Lion King, probably other stuff.
I guess I first saw him as Dave Rygalski on Gilmore Girls, and he was really cool there. Then of course came The O.C., and I gotta say Seth Cohen was the best part of that show. And there was Mr. & Mrs. Smith, and Jennifer's Body, and an episode of New Girl, and a very small role in The Ring.
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I know him from Starship Troopers; and a couple episodes of Lost; and Cowboys & Aliens; and Sleepy Hollow; an ep of The Flash; a small role in Hail, Caesar!; and probably some other live-action work, but I mostly know him for his voice work.
I know him from Reservoir Dogs; and Fargo; and The Big Lebowski; and Ghost World; and Art School Confidential; and Escape from L.A.; and Big Fish; and some voice work.