Bloxorz

A friend sent me the link to this game. It's pretty addictive. The idea is to move the block into the hole. There are various twists, such as buttons that raise/lower bridges, floor-plates that collapse if the block stands on end on them, and floor plates that split your block into two and teleport the two halves somewhere else. It makes for a nice challenge with a few simple concepts.
You can even "save" your game by writing down the passcode in the upper-right corner of the screen so you can return to that stage later on. This game is not safe for work because of the number of hours you will lose playing it.