When it comes to map mashups there's no doubt that Google Maps leads the way. On ProgrammableWeb we list 1,964 Google Maps mashups, almost four times as many as any other API. This popularity makes utilities built on top of Google Maps especially interesting, such as this contextual menu control, which pops up a menu when the user right clicks.
Gowalla, Foursquare, Yelp, Facebook Places, the list goes on. There's certainly no shortage of methods to tell people where you are and what you're doing while you're there. Startup Whatser, built on the SoGeo API for location-based apps, has jumped into the ring with a different approach to check-in services: to be useful.