Line: Android/iOS game.
Has a big userbase on Android, but nearly as much on iOS. Made around 3k$/month for a couple of months a year ago, now it's close to nothing as I stopped developing it, don't really now why. The whole thing is made with LibGDX for Android, using the (now defunct) RoboVM for iOS.
Link: http://2121.io/
Email me at nuno @ domain in link above if interested