Foursquare, the hot social network expansion that... does... social things... yeah, actually, that's sort of the problem. Sensing an opportunity to make the "check-in" utility for Twitter "more compelling for long-term use", some people have used it to turn New York into a giant game of Risk.
Foursquare, if you don't use it, employs GPS-enabled smartphones to check you in at places like bars and restaurants. What World of Fourcraft does is invite users to pick a faction (the five boroughs), and then checking in somewhere else in New York amounts to putting a plastic soldier on the map in Risk. An algorithm then determines which faction controls the area.
Right now World of Fourcraft, built by a seven-person team, has about 100 players. Mashable spoke to one of the developers, who said he's thinking about adding a levelling mechanism that makes power users' check-ins count more toward claiming territory for their faction.