The Castle Doctrine is a massively multiplayer indie game about home defense, burglary and the lengths a desperate man will travel to ensure his family's safety. The Castle Doctrine comes from Jason Rohrer, the developer of Sleep is Death, and it's available now for PC, Mac and Linux as a public alpha for $8, half off its eventual full price.
The game has no DRM, and one purchase includes access to all future updates and a lifetime account on the main game server. The Castle Doctrine is open source: Each purchase comes with PC and Mac builds, and the full source code bundle that can be compiled on Linux. The source code bundle includes the server software, enabling players to establish and run their own servers, should they wish.
Check out The Castle Doctrine (and check out, if you like it) right here.