POSTAL

Take control of The Postal Dude in his infamous first outing as he battles to survive in a world gone mad! POSTAL is a classic isometric shooter filled to the brim with fast-paced explosive action. Crazed gunmen out for your blood await you around every corner, and your only choice is to get them before they get you. Fight back with a destructive arsenal as make your way through a violence-stricken town.

It was also made open source, find the source code here.