Two impressive GPLed Open Source libraries for the use with Flash:

Flade is a physics engine written by Alec Cove. Check out the demo to see what it’s capable of.

3D Flash Studio is a realtime 3D rendering engine written in Actionscript 2 by Antonin Stefanutti, which sports an exporter for 3dsmax scenes. The rendering mechanism which becomes visible at low resolutions (nice feature) utilises a very clever way of getting around Flash’s inability to access individual pixels, reminiscent of the line-filling techniques of olde.