Action DooM 2: Urban Brawl. It's free, it doesn't require Doom 2 to run and it's really, really good. We swear.
You play a washed up cop who's forced by tragedy to take to the streets and fight through an army of thugs. The story is related to you through Sin City-inspired cut scenes that have some of the best writing you'll see in a game this year. Yeah, we know, we found it pretty hard to believe too.
As for the actual action, it's a beat 'em up/FPS hybrid, somewhere between Double Dragon and Condemned, with a colorful visual style that really pops despite the graphical limitations.
We should say up front that it's pretty hard and doesn't make a lot of allowances for modern FPS conventions (you will, for example, have to press a button to enable mouse looking) but the whole package is so shockingly good we hope you won't let those hurdles keep you from the goodness within.