I'm a big fan of Foursquare and while I like leaving "tips" at the places I check-in at and earning badges such as "I'm on a boat", I've always wanted more integration of a rating system for the places I've explored kind of like Yelp offers.
check-in for rewards
iPhone and iPad apps tagged "check-in for rewards."
5 Ways to Make Money With iPhone Apps
This list of money-making apps turns the tables on how to make money in the App Store. Instead of you paying for the app, how about letting the app pay you?