My brother and I have built a couple of test prep sites for UK medical school entrance exams: https://www.bmat.ninja, https://www.ukcat.ninja . BMAT Ninja made about $25k this year (up from $16k last year) and UKCAT about $10k.

Didn't do any work on marketing really so it is pretty much passive, but I think with a bit of work they could both do a lot better. It's a very seasonal market - we only get customers for about 3 months per year (the 3 months leading up to the exams), so thinking of branching out into other exams that occur during other times of the year so the income is a bit more steady.