I'm searching for an open source Android chat application that shows nearby users, either as List View or on Google Maps using GPS.
To be specific, an app like Peek@U.
If I have understood your requirement correctly you are looking for something like Tango. You can search for users near by and you can also contact people you already know and have their numbers. You can check it out. I honestly haven't evaluate all of it features, I've only used it for calls and video-calls with people I know. Haven't tried the "near me" option.