Bravest Warriors, and I'm blown away. It's by the same people that brought us Finn and Jake, except it's on Youtube rather than Cartoon Network.
Where Adventure Time is a cartoon zeitgeist of Gamma World and Dungeons & Dragons, Bravest Warriors is a channeling of Traveller and Star Trek and Star Wars and Douglas Adams and Futurama and Cthulhu and four-member comic book hero teams and . . . oh, just watch the frikkin thing.
Now, I should warn you - if you want to sit down and watch this with your 6 year old . . . yeah . . . no. Adventure Time had a lot of adult themes that would go over the heads of younger kids. Bravest Warriors just slams everything in your face in a glorious, wonderful, what the fuck kind of cartoon are these people making??? kind of way.
Each episode is about five minutes, so that whole 'attention span' thing is not necessary.