Improvely (https://www.improvely.com) and W3Counter (https://www.w3counter.com) have grown to $45,000 MRR.

W3Counter is completely passive -- no new code or features in over a year, no customer support load, autoscaling frontend (EC2) and backend (Aurora). Improvely gets feature updates a few times a year and has some light e-mail support load.

I also added a single banner ad to each of my open source projects' documentation sites, and that's added ~$200/month via AdSense. Developers are surprisingly lucrative targets for advertisers I guess.