Foursquare has added Radar as a feature to their mobile app. If you follow a list or if you have a place on your to-do list that is near-by, then Radar will notify you about it as a reminder. Foursquare’s Radar feature works on iOS 5.
“If you?re driving home and three of your friends are getting together nearby, we?ll tell you so you can meet up. The app doesn?t even have to be open, it just works. We call it foursquare Radar, because it finds things nearby that you normally wouldn?t know about,” said Foursquare on the company blog.