Natural Motion

Natural Motion Endorphin is slick. It lets... Never mind, read it for yourself,

"endorphin 2.7

endorphin is the industry's first Dynamic Motion Synthesis software.

At the heart of endorphin are its Adaptive Behaviors, which - unlike animation data - are completely interactive. With Adaptive Behaviors, 3D characters essentially animate themselves. Move two Football players close to each other and one will automatically tackle the other one, realistically trying to grab hold of his legs and bringing him down .

Or not. It really is up to you. endorphin allows animators to direct scenes in real-time in a way they have never been able to do before. You can change parameters or change behaviors and see the results instantly on the screen in front of you. When you're done, export animation data in one of the many formats, ready for use in your game or VFX shot."
Wow. They are using this thing in Grand Theft Auto 4, Starwars Force Unleashed, Indiana Jones and some random football games. You can download a Education version and make stuff like this video...

Unfortunately it's Windows only; but I can dream.