I help elementary schools place students into classes. classplacer.com. This involves some clever algorithmic work and turns out to be a very interesting mathematical problem. I only work with two schools since I don’t have time to expand this and I’m right around that $500/month amount but it’s more like $6k-ish per summer. It still requires some active attention on my part but maybe 10 hours of work per summer since most of it is automated.