Plague is a game I've been playing for a while now... downloading a new version each time its author Benedict Carter does a new release.
It's not a finished game, the author still has lots of plans for its future, but it is quite playable and fun. It's an overhead game, which limits your line of site on a very large map.
You play in a team of survivors against zombies! It has randomly generated maps, so each game is slightly different. You need to run around to try and kill all of the zombies, and zombie hives(which create new zombies). You start off with no weapon, amongst a group of other survivors. All of you run off trying to grab weapons and ammo (which are conveniently lying around), as the zombies move in to try and eat your brains.
There's a selection of weapons, from a pistol, to a mini gun... as well as different grenade types. Which you can use to burn things -- trees, zombies, or even your self.
If you burn yourself, ricochet bullets off walls into yourself, or a zombie eats a chunk of your brain... you can easily find a MedKit to heal yourself.
It seems you can finish a level in around 10-30 minutes if you're lucky.
Plagues graphics, and gameplay have been improving with each release... and they can only get better.
You'll want python, pygame, numpy, and psyco(optional but recommended) to play this open source game. It runs on windows, linux, freebsd, Mac OSX, and other operating systems.