$30K per month from https://www.improvely.com
A few thousand per month from https://www.w3counter.com
A few thousand per month from https://www.w3counter.com
The only downside to working alone is being on-call 24/7/365 for the servers. I have a good alert system that will wake me up if anything goes wrong overnight, though most components of the stack can fail without taking down a site. Lots of load balancing and warm failovers at a second host just in case.
W3Counter's always been a pet project I built for myself, back in the days before Google Analytics existed, that I've just been happy to see pay for itself. The site turned 10 this year.