Away3D Lite

Away3D Lite is an amazing, but Flash 10 only, achievement:
http://away3d.com/away3d-lite-v1-0-fastest-and-smallest-3d-engine-in-flash

Matthew Casperson has a lot more on it here:

http://www.brighthub.com/hubfolio/matthew-casperson/articles/49048.aspx

Very, very useful for creating interactive 3D in banners etc. And it's a lot faster than the full blown Away3D engine (because of the "native" 3d support in Flash 10).