Free Portal
 

The Valve game, Portal, is free for PC and Mac users until May 24th.

It's essentially a puzzle game, but from a "first person" perspective (which means it looks like all the shooty games out there at the moment). You use a portal gun to open up a wormhole to get you out of each room.

It's basically nonviolent, but there are levels later on where there are turrets to bypass which will shoot at you. The beauty of the game is that it makes you think.