http://www.miniclip.com/games/bloxorz/en/

The basic concept is very simple -- you've got a 1x2 sized box which you need to roll around a grid without falling off the edges, ultimately reaching the exit. It is unusual because the orientation of the block really matters -- vertical or horizontal -- and you can't change orientation without rolling across to the next space, which could make you fall off the edge! You also need to hit certain switches only when you are vertical, and you can only exit the level when you are vertical.

This makes it very interesting because you often have very confined spaces in which to turn and move, and you often need to carefully avoid bad switches which will make crucial bridges disappear.

Great fun, and brilliant for dipping in and out of whenever you've got a few minutes free.