I built an addon for a .Net testing tool, TypeMock Isolator. I stuck a deal with them that I'm doing development and they're doing sales. Got some money which were pretty good for me back then (2008-2010).
The bad thing was that I was disconnected from my end users and couldn't do anything but guess which features they needed (I used it myself). The sales declined, and when it became about $300 / year I said OK let's discontinue this thing.
My second one was a total disaster. An online C# coding tool.