[iluvmylife]

I made a super simple paid Android app with a list of interesting Physics Puzzles (https://play.google.com/store/apps/details?id=com.boredominn...). Took a month to make and now it is completely passive. Brings in some spare change with zero maintenance (~$50/month).

I built this in 2011 to learn app development (its a webapp built using PhoneGap). Took about a month of evening/weekend work to push out, and most of that time was consumed by collecting and creating interesting puzzles. It was featured on Google Play's Top Paid Educational Games leaderboard for a while, and that contributed to a spike in income. That apart, I haven't done/don't know of any viable means to promote it.