Rocko's Modern Life is still cool to watch. It's weird, because it has everything I normally hate in a cartoon -- unappealing characters, ugly animation, sit-com embarassment-based humor... for some reason it works, though.
Angry Beavers is... interesting. In a sort of 'Beavis and Butthead' way, although not quite as stupid.
Invader Zim is, of course, pretty cool, but everyone knew that already.
All three of those cartoons are played incessantly on Nicktoons. }:P
...but man, Black Powder War is depressing.
Oh! Also, Pirates of the Carribean: Dead Man's Chest, I saw that yesterday... it was pretty cool, but definately suffered from being a sequel. I totally want to join Davy Jones' crew, though -- eternal life AND free tentacles? What's not to like?