In 2006 while I was still freelancing, I suspected that creating a digital icon set might be a good way to have an actual product that generated a bit of recurring income.
Over the next few years, I built up multiple sets within the same visual icon family, almost 600 base designs available in 3 different bitmap sizes — which was how icons were commonly delivered at the time.)
In order to assist with finding the right icon in a set that large, I also spent time coding up a textual search engine that would match visual features and the object portrayed. This allows an owner of a set to search for things like things with letters on them, blue arrows, and orange flags.
While never a smash hit, the larger icon family sold relatively well over the years, more than paying for the time I spent creating it. I made a few free sets for promotional purposes (most notably an iPhone theme that departed from the style set by the rest of the family) but in 2011 I threw a Creative Common license on the entire family and made it freely downloadable, with license fees only applying on commercial use. To date, the various sets have been downloaded 400,000 times.