I won't get super into the details, but... A friend and I co-founded a small startup / lifestyle business in a niche hardware / hobby market. Built a platform around it. It's now inching close to 6 figures in 6 years. Not earth shattering, by any stretch of the imagination, but it's more than paid for itself and our time investment. Especially considering that my time investment as the primary developer is somewhere around 10 hours a year for maintenance purposes.

We grow the user base slowly, organically, deliberately. Our quite reasonable cost can be a barrier to entry for some users but our particular solution to this particular problem can save people many thousands of dollars over the course of a few years.

edit/ Typos...