I know him from Smallville; and Dark Angel; and and Supernatural.
I know him from School Ties; and Mallrats; and Chasing Amy; and Good Will Hunting; and Shakespeare in Love; and Dogma; and Daredevil; and Jersey Girl; and Hollywoodland; and Batman v Superman; and Dazed and Confused (though I didn't recognize him in that); and Jay and Silent Bob Strike Back.
Best known from Seinfeld, of course. But there's also Pretty Woman; and Hachi: A dog's tale; and Grow Up, Timmy Turner; and Rodgers & Hammerstein's Cinderella; and he appeared as himself in an episode of the Mad About You revival; and some voice work.
I know him from 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.
I know him from 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 The Santa Clause 3; and Indian Summer. And he voiced Schmendrick in The Last Unicorn.
I know him from The Chronicle; and Supernatural (starting in season 8); and a recurring role on New Girl (starting at the end of season 2); and an episode of Stumptown; and undoubtedly various things that I don't remember. Also some voice work.
I know him from Arrested Development; and Running Wilde; and When in Rome; and a recurring role on 30 Rock; and Series 7: The Contenders; and Muppets Haunted Mansion; 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.
I know him from Buffy the Vampire Slayer; and Double Rush; and the Scream franchise; and Muppets from Space; and The Adventures of Sharkboy and Lavagirl; and a few eps of Pushing Daisies; and Sigmund and the Sea Monsters.
Okay, I'm vaguely aware of him as Lou Grant on Mary Tyler Moore, though I never really saw much of that. But I know him from an episode of The X-Files; and a recurring role on Studio 60 on the Sunset Strip; and 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 a very minor role in Gremlins 2. 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.
I know him from The Goonies; and the Lord of the Rings movies; and an episode of Masters of Science Fiction; and The Colour of Magic; and season 2 of Stranger Things; and an episode of Brooklyn Nine-Nine. And voice work in Stan Lee's Mighty 7; and Justice League: War.
I know him from Ghostbusters; and Real Genius; and a minor role in Oscar; and the second season of Defiance; and surely other things that I don't remember.
I know him from Blackadder; and Mr. Bean; and The Curse of the Fatal Death; 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 The Wild Wild West Revisited and More Wild Wild West; and a minor role in Inspector Gadget; and a minor role in Batman Forever; and Boston Legal; and a couple eps of 1600 Penn; and Geppetto. And some voice work.
I know him from Voyage to the Bottom of the Sea; and a bunch of Beach Party movies; and a minor role in Pajama Party; and Ski Party; and Fireball 500; and a minor role in Grease; and Back to the Beach.
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 a minor role in Pixels; 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 know him from Mad About You; and Mystery Men; and a small role in Now and Then; and Along Came Polly. And I must have seen some of his other work, I'm not sure....