PTLens
In 2000 I noticed some obvious barrel distortion in images taken with my new Fuji digital camera. I knew how to fix distortion with PTGui, but the interface is not exactly user friendly. To remedy the problem I wrote a program so that you could press a button and distortion would be corrected.
People would send me images that depict distortion. I would then use their images, and PTGui, to determine coefficients that would correct distortion. The coefficients were saved in a database for all to share. Named the software PTLens and sold it for a mere $25.
Twenty years later there was little demand. This was primarily due to the fact that people started using their cell phones to take pictures. They weren't exactly interested in distortion correction. In addition I was occupied with other projects. Consequently 2020 was the last year I sold PTLens.