You know Instragram, right? Not only is the app fantastic, but they also have a pretty kick-ass API. It’s read-only but that’s all you need to start building an app with it.
Bonus: After putting photos on the map, I realized it would be nice to just see what’s currently popular on Instagram. Again, there are plenty of sites around that show Instagram’s popular photos but I had fun building my own version: instapopular.