While this won't key-frame like Reactor, it has interesting capabilities for real-time applications.
Users of Autodesk 3ds Max have newly-updated downloadable plugins available for use with NVIDIA’s PhysX physics SDK. These plugin downloads include full source and user guides.
The PhysX Plugin for Max has been upgraded to version 1.0.3 and provides support for Max9/Max2008/Max2009 with NVIDIA PhysX SDK 2.7.3, 2.8.0, and 2.8.1.
Code:
http://sourceforge.net/projects/physxplugin/
http://developer.nvidia.com/object/physx.html