BrightKite is a useful tool that allows users to "check in" and send their locations to people following them on Twitter. Its goal is to allow people to meet other Twitter/BrightKite users in the same city, area or block. If users aren't comfortable sharing their updates with strangers, privacy controls allows users to change what updates are seen, by whom and how specific (eg - even though one may check in within a block radius, non-friended people can only see what city the user is in).
BrightKite also allows users to post pictures based on location. It also has GPS functionality, so if a user clicks on "Guess my Location" from your phone, it can zero-in on their location without them having to input anything.
Try it out!