Urban Defender has gangs of players putting down the guns and knives and instead reaching for what looks like a glowing dodgeball. The objective here isn't to go after other people, though. You're aiming for buildings.
The game apparently isn't quite finished yet so the multilayer backend I'm about to describe isn't in place yet. Ideally, you should be able to take your ball around the city and toss it against buildings, claiming that structure for your team. The ball has an accelerometer inside and is GPS-enabled so that it can upload impacts to a map on a server and mark your territory. That part works, but you can't score points or play a "game" par se — yet.
The ball also acts as your guide. It'll vibrate and pulse differently when you're in enemy or neutral territory, and make noises as you claim an enemy structure. Check out the video below for a demonstration.