Subvein is a game still in development but it looks like a ton of fun! From the Subvein website:

Subvein is a free multiplayer action game. It has over 20 weapons, a strategic skills system, crazy vehicles and countless user-generated maps. It’s tactical, fast-paced and most importantly, fun! Download and play Subvein now – it takes mere minutes to setup and best of all it’s 100% FREE!

And from the creator of Subvein:

Basically, Subvein is a top-down multiplayer shooter. It was heavily inspired by games like GTA, Counter Strike and Diablo. It has guns and vehicles, all the standard game modes like CTF and deathmatch, and a rather sexy skills system. There are 33 skills to choose from including stuff like Regeneration, Invisibility, Bullet Time and Force Fields. Basically you run around completing objectives and shooting eachother, and as you kill enemies you’ll level up. This gives you a skill point that you can invest into a skill and so on.

Check out the promo video.

On Reddit, the developer discusses a bit about how he made and why (inspired by GTA2).

When I get home I’m definitely playing this.