I need a 3d environment to write some artificial intelligence code in, like a city with cars and people that walk around randomly and a decent physics engine. A grand theft auto type place. Is there anywhere I could download an open source game demo and start from there instead of having to make physics and design a whole city myself? Oh yeah, and I would like to write my code in c++ or python preferably, but i'm flexible.

I would recommend Unity, or a similar authoring tool such as Shiva, Blender Game Engine, etc.

For the free version of Unity, your options for scripting are Unity &#106avascript, C#, and Boo. I know very little about Boo, but my understanding is that the syntax is Python-like, so that might work for you. You could also try C#, the syntax of which is somewhat similar to C++ (it is, however, a very different language than C++).<br><br>There's also Panda3D. I don't know much about it, but my understanding is that scripting is typically done using Python. I don't know that it offers an integrated development environment like Unity or Shiva, and I also don't know whether it has built in support for physics, but you should be able to find those things out easily enough.