Behave

The Behave project is a system for designing, integrating and running behaviour logic using Behaviour Trees for simulated characters in unity projects. Find a complete feature list and a more in-depth description in the Documentation section of this page.

Not sure what a behaviour tree is or why its so neat? I recommend viewing the first four videos (from the bottom up) at aigamedev.com.

Not yet aware of the existence of the unity engine and all its awesome-ness-ness? Check it out at unity3d.com.