This just came out for the DSi, and is pretty awesome to mess around with. I love drawing animations, especially since I don't have the money to afford a tablet, and most programs like that don't show everything you've drawn up to that point. It's also free. If you have a DSi, you should download at once.
EDIT: You should go to the website flipnote.hatena.com. Holy crap, there are some crazy American entries! It's absolute anarchy and madness. There's one that's a picture of the CD-I Zelda game, with "DINNER" repeating over and over again. I've seen the Earthbound boss. There's a Geico ad, for Christ's sakes. This is the most insane thing that's happened to a Nintendo portable handheld ever.
EDIT 2: After seeing some of these, I've decided this is the WORST thing Nintendo has ever come out with. This power is too great for some. Eyes may bleed, and ears may burn.