This looks pretty neat, it's an opensource 3D library written completely in actionscript3, for flash9+. It's the first serious 3d engine I've seen in flash, and while it is still at an early stage (somewhat closed beta, if you join the mailing list you can find info on how to get it), it looks like it will be amazing. The coding is clean, and there is already some documentation (often rare in opensource projects).
Here is a quick test with some Bouncing Cubes that I've made:
(requires Flash 9 player to view)
(click to randomize velocity) main source file