They've got all 255 episodes of "King of the Hill", plus the first seven seasons of "Family Guy" and all the regular episodes of "Futurama", plus the four movies. Also, they've got tons of Nickelodeon series, from the classic ("Rugrats", "The Ren and Stimpy Show", "Rocko's Modern Life", "Hey Arnold", etc. ) to the contemporary ("SpongeBob SquarePants", "The Fairly OddParents", "Invader Zim", "Danny Phantom", and so on). And from Disney, they've got nearly every episode of "Phineas and Ferb".
Surprisingly, no Cartoon Network shows are available for streaming yet. I'm sure it's just a matter of time, though.