Mashup games are always fun. Those built upon FourSquare have an extra layer of enjoyment, because they're tied to the real world. In the case of City Warfare, players use various water weaponry to soak other FourSquare users.
First, you check-in as you normally would using FourSquare. Then, open up City Warfare in any browser and connect to FourSquare. Now you can squirt your water pistol or buy balloons of various types to place at your current location. Check back later and see how many people you soaked.
City Warfare won the Location Wild contest, a pretty cool feat given it was battling for "most interesting location-based app." It seems like a good choice to me. It's fun, it's casual and it's just a little bit addictive.
See other games in our collection of 121 game mashups.