Foursquare Check in to your favourite stores, restaurants, events and more--get mayorships and discounts, as well as tips & reviews from other patrons who have been there before.
Gowalla Much like Foursquare, check into local businesses and locations.
Facebook Facebook now has location-based check ins in its app, called "places."
GetGlue You don't have to just check in to a location -- use the social entertainment site to check into your favourite shows, movies and more. Find out what your friends are enjoying, and share your favs.