This game, Alien Swarm, just came out recently and is free to download and play. "Alien Swarm is a free top-down shoot-em-up by Valve Corporation. It is a remake of the Alien Swarm mod for Unreal Tournament 2004"

http://store.steampowered.com/app/630/

Valve also released an SDK including buildable source code alongside the game. Just reading in their forums that lots of mods are getting started: 32 player deathmatch mods, first person shooter mods, etc.

I have a feeling there is going to be some kick ass stuff coming out for this game for 2 reasons: the game is free, Valve seems to support their games quite a bit more than their average devs.

I'm going to give Alien Swarm a try. While top down shooters arent my cup of tea I am definetly keeping my eyes peeled for cool mods.