Foursquare is from NYC, Gowalla is from Austin. Foursquare emphasizes nightlife, Gowalla gives a bit of a nod to travel. Both are location based check in games that try and make a competition out of activities in real life.

One thing that I really like about Gowalla is the icons... they seem to have a custom icon for just about everything.

Gowalla is lagging Foursquare a lot in adoption, but it's still very, very early in the game.