Banjo is either an amazing app or a really creepy one, depending on how you look at it. The app finds your Facebook, Twitter, Foursquare and Gowalla contacts, puts them on a map and pings you when they're nearby.
Unlike previously mentioned Windows/Linux app Creepy, Banjo offers something beyond just stalking people on your social network: It can help you not miss the people you would want to meet up with. For example, if you go somewhere like the shopping centre and your friend happens to be there at the same time but neither of you are aware of this, normally you'd just miss each other. Banjo, however, automatically alerts you.
The automatic part is key. You don't have to check into Foursquare or look for people by launching the "Find My Friends" app or similar apps. Your friends would have to be sharing their location data on these networks, naturally.
Banjo lets you filter alerts by contact and will be able soon to set the radius for the notifications too. You can also tweet or check in to social networks from within the app and send a chat message to a contact ("Hey, I just saw you're in the neighbourhood, let's have lunch" or something more sinister; it is Evil Week).