Surprised Simple Analytics[1] hasn't been mentioned yet. I don't know how profitable it is but this seems like a great example re: the question.
I do some freelance work on the side. As one hour of freelance work is more or less equal to one customer for a year it’s still way more satisfying to work on Simple Analytics.
I try to keep it within 40 hour work weeks but the reality is around 2-10 hours freelance and 30-50 on Simple Analytis. But it’s sooo fun!