Almost a year and a half ago, I wrote up a small Unity3D project that demonstrated a few basic AI behaviors (patrolling, wandering, following). You can check it out here. I get more emails and questions about that simple project than anything else on this website - I guess simple projects that demonstrate the basics of game AI are something people are interested in!
Anyway, this project had a major problem – the sample NPCs didn’t deal with changes in terrain height well (or at all). This has been bugging me for a while, but I haven’t had a chance to fix it until tonight. Check out the project page for a new version, complete with NPCs that don’t burrow into the ground at the first sign of a change in the y-value of the terrain underneath their feet.