Merlin's Squares
This is more of a puzzle than a game, as there is only one player. In its simple form it consists of a 3 by 3 grid of squares, each of which is either blue or green. The object is to make all of the squares green except for the center one. The squares correspond to the number keys on the number pad. If you click on a square or press the corresponding number, some of the squares toggle their colors. Clicking a corner toggles that corner and the 3 squares next to it horizontally, vertically and diagonally. Clicking a side square toggles all the squares on that side. Finally, clicking the middle toggles everything but the corners.
For a more difficult puzzle, you can play on different surfaces or with other numbers of colors.
In all such games, the object is to turn the grid a solid color. The standard game is on a plane, and on a plane you can choose between 2, 3, or 4 colors. You can also play on a torus or a Klein bottle (each of which only works with 3 colors). For the other shapes everything works like the middle square does on a plane, but the board wraps around top to bottom and left to right. For the torus the wrapping is easy, but for the Klein bottle, horizontal wrapping is normal but you flip right and left whenever your wrap vertically. Try experimenting with the controls. To switch between surface, use the P, T and K keys. The + and = keys change the number of colors if you are on a plane. If you get stuck, press H and each square will be labeled with the number of times it must be clicked in order to win. If the keys don't work, click on the game to restore focus.