I incorporated right out of university. The business was Debuggex Inc., and the first product was a regex debugger. This was an ultra-niche (I didn't realize just how niche at the time) tool to help you understand regexes. A regex is like ctrl+F on steroids - it lets you find not just specific pieces of text, but complex patterns.
Suppose you wanted to find all the dates in a document. You might want to match any digits that look like YYYY-MM-DD. To solve this, you can use a regex like this: \d{4}-\d{2}-\d{2}. A few squigglies, but overall, not terrible.
WTF is this s#!t? debuggex.com was supposed to answer that question, and it probably did the best job of any available tool by drawing a cool animated diagram to let you "step through" and see how the regex worked. This was my first experience building a "tool for insight", and I was hooked!
But it was a sucky business. I sold a single $4,000 license in the 6 months after launch, and had roughly $25/month in subscription revenue. People were using it, but they weren't willing to pay. Today, I'd estimate that the total market globally is maybe a couple million bucks a year. In the decade since I launched it, I've only ever personally used it twice. So the need just wasn't that great.