SongStumblr is a nifty new app that lets you see what nearby users are listening to. The developers call it "geosocial" music discovery/sharing—SongStumblr lets you see and hear your neighbors' music, as well as broadcasting your own. Here's what the developers have to say about the app's functionality:
Users can listen to discovered songs from other users for free, purchase songs, or find information about songs from online databases. Users can control their own music which will be shared, right from within the application, with an inbuilt music controller with customizable colors, or by running the application in the background and using the default iPod™ App. (Although being an iOS 4.0 feature, it will run on 3.0 and above.)
Basically? Share your music, and see what's playing nearby. Because it's Bluetooth-powered, you don't even need an internet connection to hear what nearby users are listening to. If you see a song you like, you can add it to your database for later reference. SongStumblr can also display your saved songs on a map, to give you an idea of what you've heard and where.
Of course, SongStumblr's usefulness is tied to how many users are broadcasting their songs...so it should come as no surprise that the newly-released app is currently free. If localized music sharing and discovery sounds interesting to you, go ahead and give SongStumblr a try.