Portal is a great game that comes with the much anticipated Orange Box. The game is a pretty inventive one where you use a gun to create portals on the walls, ceilings, and floors and then pass through one portal to get to another.
Well a flash version of Portal has just been released and I think I finally found a game that will kill my addiction to Desktop Tower Defense (yes, I do still play it). The game is not exactly the same, this one is only 2D but it still is really fun.
Go ahead and try it out though, it will most likely easily kill an hour or two.