http://teamsite.io - Over 6 months working evenings and weekend and am almost done... It's an existing space so I needed to have all the features of my competitors upon launch.
I made an app called BHive Tasks. I did it originally for BlackBerry since I saw there was very little competition there. On a good day it would make $150/day ... and it's probably made 40K so far.
I also tried to make the same app for Android, Windows Phone and BlackBerry 10... The Android app has done ok (a few thousand dollars so far). The windows phone app was a complete disaster. Even though it was at one point #3 in productivity, it was only getting a few sales a week (I decided to never developer for windows phone after that).