I mentioned it probably a year ago, but the Citrus Engine is finally officially live. You can check out the site, which is also a demo of the engine, at http://citrusengine.com.
The Citrus Engine is a sidescrolling platform game engine built in ActionScript 3, making it easy for designers and developers to "get right to the good stuff", as it were.
Let me know if you're interested in checking it out. If you're a student or teacher, you can grab all the code for free.