I've never heard or Core Wars before, but it looks interesting. I do have to vouch for RoboCode, though. That's fun and challenging, especially if you have a group of people competing against either other.
There's racing car simulator game TORCS also where on top of the typical end user playing it (you actually "driving" the cars), you can program robots which control the cars. Regular races are held between robots created by different people.
Although there has not been a lot of activity surrounding the game over the last few years, there was a tournament held recently, and there is a yahoo email group.
My favourite was 电脑机器人 back in the 90's - you could write your bot in pretty much any language that could compile a DOS executable. Still runs quite nicely in DOSBox :)
It's usually easy for new programmers to pick up on languages like C++ when you have a strong understanding of Java basics. Colobot allows you to program automated craft using a language almost identical to Java and to move, sense, and manipulate their environment in order to accomplish missions on a dangerous planet.