It doesn't exactly qualify as a one-man business, but I'm running a few side projects bringing around $500/month:

- https://batgrowth.com: A website monitoring the growth of The Brave browser / Basic Attention Token, in terms of publishers adoption (Fun fact: Wikipedia founder, Larry Sanger, tweeted about the site)

- https://ethereum-cours.com: A CMC clone built mainly for fun, bringing some decent referral revenue

- My tech blog

I've built many more side projects, most were earning $0/month after a few months, but I've also sold 2 side projects, so overall it's worth the effort