Flipnote Studio is now available for free in the Americas, and it rocks. I would suggest dropping everything (except your drawers perhaps) and getting your download on asap.
If this is the first you’ve heard of Flipnote, its a free piece of software available for download to DSi’s everywhere that allows you to make brilliant little animations complete with sound, classic sprites, pictures, etc. Once you’re done you can upload them to the web (here) or share them locally. I haven’t had time to sit down for more than 30 mins with it, but I can already see this becoming a time sucker. Just browsing the site is too much fun.
