There’s a new kid on the (crowded) block. Another 3D engine has been released, but this one is worth a serious look. It’s a new lighter version of Awayd3D called Awayd3DLite. I have been playing a little around with this engine, and for my purpose it’s just as good as Papervision, but compiling time and filesize is reduced tremendously – so I’m probably using this in the future! The engine has primitive objects such as cubes, planes and spheres, so unless you are doing really heavy/advanced 3D development, this engine is highly recommended.
Note that you must publish for Flash Player 10 to take this engine into consideration (but you are already doing that I hope…)
Here are some other links for my favourite engines – or at least the ones I’ve tried 
Five3D
Papervision
Alternativa
Btw. my next project is using my own “engine” for simple 3D stuff like cubes and z-sorting.