I'm embarrassed to admit it, but I've seen every Sex and the City episode through season 4. I kind of lost interest though, so I haven't bought the remainder of the series on DVD yet.
One day, assuming they keep cranking out the DVDs, I'll have seen every Simpsons episode.
Soon, as soon as I complete my DVD purchases of Futurama, I'll have seen every episode of that.
When I was still living at home, MTV replayed all the My So Called Life episodes, so I've seen all of those.
And I'm absolutely certain I've seen every episode of the original Star Trek. I've only missed a few TNG episodes, too.
I don't know about anything else though--I don't watch all that much TV, so that's probably it.
EDIT: I forgot about News Radio; it was shown in syndication for a few years, and I taped it most nights so I would have something to watch after work.
Come to think of it, I've probably seen all Cheers episodes, but I can't be 100% sure. (That was in syndication for years and years and years, so I've seen most of them three or four times.)