I made a connect-four game to demonstrate how to make a decent computer AI quite easy.
I've included both the executable (windows) and the source code. It should be quite easy to port it though, because I've put everything non-standard in one file (clrscr) for the user to change.
The AI uses an alpha-beta algorithm (described here). I've made a very simple evaluation function that only tries to move at the middle of the board. The AI would probably be much better if I enhanced the evaluation function, but even now, it's decent.
Please try the program and report any bugs (it didn't take very long to write so there's certainly room for improvement.
EDIT: I managed to forget some header files.
EDIT: Updated version with the possibility to reduce the AI