My most widely-used project (that I was solely responsible for) is an absolute horror of tangled code. It was originally written -- or rather, hacked together with spit and baling wire -- purely for my own use, as a tool to accomplish specific tasks.
Turned out some other people thought it was interesting. I made the code available -- and nearly 2 decades later, it's still going strong, largely thanks to others who have stepped in to handle the occasional maintenance tasks.
The code is still appalling; but it does its job, and people still value it.