Robocode
http://robocode.sourceforge.net/
It's all done in Java instead of C, but the two languages are very similar (close enough that you'll encounter few, if any, problems).
You program a robot to autonomously fight other robots in an arena. You can upload your robot to the 'net to try it against other people, download their robots to modify, all kinds of great stuff. I spend at least 30 minutes a day working on my 'bot.
The programming is a bit different from the way it's done it FIRST, but it's still good for practicing.