Flipnote Studio is a free download via DSiWare. It gives you a notepad with hundreds of pages, three coloured pens and an eraser. You can also import DSi photos and then manipulate them as part of your animation. And you can use the DSi microphone to add music, speech and other sound effects.
Your clips can be uploaded to the Flipnote Hatena website where they can be shared and rated amongst the community.
Or, if you prefer, Flipnote can simply be used as a regular memo pad. The only question is: how long until the first stick figure porno is made?