Foursquare, Yelp, and Gowalla are all mobile apps that track location-based check-ins, often with added info like reviews, pictures, or status messages, but the problem is that they’re all pretty compartmentalized; most folks only really need one check-in network, and it’s never guaranteed that their friends are in the same one. Luckily, FourWhere has used APIs from Foursquare, Yelp, and Gowalla to put all of that geotagged information on one Google Map.
FourWhere doesn’t have a mobile app just yet, but that doesn’t seem like a long ways off. Check-in applications are becoming pretty popular lately, and with Facebook, Buzz and Twitter getting involved with place relations (as opposed to just GPS coordinates), the real social butterflies will want to post to multiple accounts at once without needing a whole home screen-worth of apps. Buzzd is currently trying its hand at this very concept, but I think they could use a bit of competition.
Check it out at FourWhere.com.