T4 is a challenging board game for Windows. It can be played against the computer or between two human players.
It's a game with simple rules, not unlike Othello/Reversi or Four-In-A-Row, but richer and more complex.
It has the interesting twist that your opponent can choose which piece you will play. (But you can take your revenge when it's her/his/its turn).
The computer opponent plays a fairly decent game, with parametrable skill levels.
The detailed rules of the game are explained in the program's help file.
T4 has won Envelope Games' DARN FINE GAME award, which, in their own words, is given to "independent game inventors whose games exhibit originality, intelligence, quirkiness, hilarity ... in other words, good game qualities".
Free software >