It seems like Sudoku is the only game in town at the moment but this game will certainly test your powers of mental agility. Ingenious is basically a glorified version of Tetris. In it, you must conquer the board through the careful placement of rows of colors. With every move, you create new possibilities. The more colored pieces are placed next to each other, the more points you can gain.
This is one hell of a challenging game but at the same time, a hell of a lot of fun. The program is remarkably clever at giving you the most challenging moments just when you need it. I imagine it's most fun however when played against a friend across a network. Many users compare this game to Tetris, Knizia and other similar abstract strategy games in terms of the challenge it presents.
The graphics are simple but it's the gameplay and challenge that'll keep you hooked for hours and hours.