Tell People Where You Are, Win Free Stuff

You've mastered Facebook and Twitter. Next up: Location-based social networking. So how does it work? Here are the details on a few apps you may have heard about: Foursquare has apps for iPhone, Blackberry, Palm, Android, and other devices with GPS. "Check-in" to let it know where you are (bar, restaurant, office, etc.), then it will tell your friends where to find you and you'll also see their favorite spots. You can also create tips about the places you go and read those left by other users--check in at a bar and you might find tips on tasty drinks from other users. Plus you earn points every time you check in somewhere, and users can also get rewards (like free drinks) from participating businesses.

Gowalla (for iPhone, Blackberry, Palm, and Android) is more like a location-based game: Players get rewarded for visiting new locations and adding new spots to the Gowalla database. They can also take "trips," checking out a bunch of themed spots to get rewards. And CauseWorld (currently for iPhone and android) gives you the option to do good just by walking into a store--you don't even have to buy anything to earn "karma points" that sponsors like Kraft then turn into real cash donations to one of 18 causes (like "care for abused animals" or "donate a book"). Think about it--you probably always carry your smartphone with you, so why not give one of these a try?