Orcs. Must. Die. Switch your brain off and dispatch them by the thousand. An enjoyable and more action-focussed affair comapred to most tower defense games, it's perfectly viable not to place any traps at all! (sometimes)
I love it.

Dead cheap and worth buying for the title screen music alone. The game combines roguelike gameplay with nerd humour and a great soundtrack. It's also friendlier than most roguelikes as it actually has graphics and cuts out a lot of the more 'dangerous' features like unidentified items. It's still happy to kill you if you get cocky, not an easy game by any stretch.

Also, check out those eyebrows! They command you to play.

Screw the Metacritic score, this is loads of fun and more stuff has been added since it was reviewed. 4 player coop just got added too - if you get it, send a message my way and let's play some!