Those of you who have the DSi, check out the DSi Store, there's a free download of Flipnote Studio. It's an app for creating flipbook-type animations, with sound. Once you finish your masterpiece, you can share it with other DSi users or upload to the Flipnote Hatena website for the entire world to see. You can also use the app to download and view other artist's creations from the website.
If you don't have a DSi, you can still check out the animations on the Hatena website.