A site I built to teach myself bodyweight fitness (from a routine I found on reddit) -- grew a userbase and built up more detailed tracking, configuration, and usability.

The MRR is a bit hard to calculate right now (since I charge annually), but somewhere in the $500 - $1000 range.

Edit: This app grew from feelings of being stuck / burned out / almost depressed that can come from being in front of the computer too much. I was trying to build another startup out of college from my parents' house, and was having little progress or social interaction. There's been a number of threads about depression on HN recently, I felt the same. I realized I needed to be able to work out, but had no good gym accessible at the time, thus this little project was born (and went on to be more successful than the app I was initially trying to work on).