Foursquare works with iPhone, Android, BlackBerry, and Palm (WebOS) phones. If no app exists for your smartphone, you can always use the Foursquare mobile Website instead. Foursquare refers to announcing your location--and thus telling your friends where they can find you--as "checking in."
You can check in to cafés, bars, restaurants, parks, offices, and pretty much anyplace else. Once your friends know where you are, they can recommend places for you to go or things for you to do and see nearby. To keep it fun, the service gives you points for each check-in; and in time you can earn various badges tracking your progress toward Foursquare elitehood.
Even cooler, if the service recognizes you as the mayor of a location (by virtue of your having visited that place more frequently than anyone else), you are in for some freebies. Foursquare has a massive list of places all over the world that offer special discounts and free drinks to their mayors, or to anyone who has registered a certain number of check-ins at their site.
Gowalla, like Foursquare, works with the iPhone, Android, Palm (WebOS), and BlackBerry (on Bold, Curve, and Storm phones) platforms. The service has a huge database of locations curated by users, and you and other participants can trade virtual items that you've collect. Gowalla also has worked out several advertising partnerships that enable you exchange virtual goods for their real-life counterparts for free.
Gowalla recently added a trips feature (iPhone-only at the moment; Android and WebOS versions coming soon) that lets users recommend up to 20 locations that they like to other Gowalla enthusiasts. Your friends can then complete the trips, such as city tours or bar crawls.
Brightkite works with iPhone, Android, Palm, Nokia, and BlackBerry smartphones. This service lets you establish two kinds of social connections: fans (Twitter-like followers) and Facebook-like friends. Aside from sharing your location, you can post short messages that your friends or fans can respond to with a thumb up or thumb down.
Brightkite has excellent privacy controls that let you share individual posts with everyone or with friends only; you can also cross-post on Facebook, Flickr, and Twitter. Like Foursquare and Gowalla, Brightkite uses the check-in system for bars, clubs, museum, and the like, and it finds your location automatically.