Built a hobby informational web app back in 2005 related to a collectible item I was interested in. It basically tells you if your X is considered common or rare (potentially valuable). Determining that is nuanced and people don't want to figure stuff out on their own, so my web app became popular.

At this point it has earned over $400k USD lifetime with 75% of that being in the last two years. 100% Adsense. The audience who find/use my site are good candidates for high-dollar ads, which is why it has done so well. New data (a few rows in a table) needs to be added once every few months, but it takes < 1hr. Realistically I should invest in blog/content but I don't really find it interesting any more.

Following the "make niche complicated data easily parsable" concept I built another web app in 2018 for a different hobby. For example if the hobby was "phones" and the physical dimensions/specs of "phones" was very important to choosing it, my web app would let you visually compare any two "phones" side-by-side at the correct dimensional scale.

It apparently solved a problem and traffic grew organically. After years of work this web app should get close to 6 figures annually in 2023 - earned via affiliate sales of "phones" and accessories. It's is a much more active endeavor, but if I walked away for 6 months I bet revenue would stay near flat. It fills my time and I find it interesting.

I'm working on web app #3 now which is tangentially related to the "phones" concept.