Toribash: Violence Perfected
homepage: http://toribash.com/
OS: Windows, Linux, Mac
Open Source: No

I've been playing around with this game for the last few days and have found it pretty entertaining. It's best described as a turn-based fighting game.
Here's the game's official trailer.
And here are a few gameplay videos that show you what the pros (not me) can do.
Vid1
Vid2

Enjoy.