You should check out Flixel. I've played with it a bit and I've found it to be really cool. Here's a few tutorials on it:
http://www.creativeapplications.net/flash/flixel-2-tutorial-flash-tutorials-games/
http://www.brighthub.com/hubfolio/matthew-casperson/blog/archive/2009/12/02/creating-a-flash-platform-game-with-flixel-and-flex.aspx