With the Gmail Python SDK by Google, developers can access Google APIs including Gmail. Resources aim to simplify API calls, handle auth with fewer lines of code, use the library with Google App Engine, and use standard tools for installation. Additional APIs that interact with this library include Discovery Service, AdExchange Buyer, Books, Cloud Monitoring, and DCM/DFA Reporting And Trafficking APIs.
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.