Hi, I am looking at learning AI, but I need a good environment to practice the techniques I learn in. I could code my own simple applications, but I thought there must be alternatives around. Perhaps popular games (UT2004, ET, Doom3, HL2, etc) that allow scripting/programming AI powerfully enough that I could employ the techniques. Or maybe programs where you can program simulated robots to fight each other in an arena (on a 2D plane I'm thinking, not 3D). Or maybe even open source games that have good AI scripting/programming interfaces. I don't know what's available or what's best, so I'm interested in hearing your experiences and recommendations - thanks.

I would recommend coding your own application. Then you can alter the world/game rules to try out new techniques.

Why coding your own is a good idea, it typically requires knowing a bit of graphics, etc. Not a bad idea, but for me personally, starting with Quake 2 and Half-Life gave me some content to work with and avoided the startup time of having to code my own app.

I haven't looked at many SDKs in a while, but you may want to give Half-Life 2 a shot as it give you a lot of freedom (ie the SDK contains a huge amount of functionality, and is in C++) and has there is a decent sized mod community to learn from. Unreal 2003 is probably another safe bet, though you will likely be working partly in C++ and partly in unreal script there.

Chess is a fun option to code. As when it comes down to more graphics related AI programming I have no advice.

AI is simple. Just figure out the way you think and throw it into code. That's how you learn applying AI to games.

For example, in Tic Tac Toe, you did your turn and now it's the computers. How should he think? How did you just think? Start with simple games like this. It helps a ton.

JRobots is nice (for those who know Java), or the older version CRobots for those people who like C. (Links are just examples I found googling).

It basically is a game centered around programming your own robot and put it into an area to see which robot (or the program controlling it) will win. Usually it comes with some example programs for the robots, so you can read them, analyze them, modify them and finally write your own.
So something I would call a good start into AI.