A popular feature on current mobile devices is GPS capability, so the device can tell you where you are at any point in time. While the most common uses of GPS services are for mapping and getting directions, there are other things you can do if you know your location. For example, you might set up a dynamic chat application based on physical location, so you're chatting with those people who are nearest to you. Or you could automatically geo-tag posts to Twitter or similar services.
GPS is not the only way a mobile device can identify your location. Alternatives include the following: