My side project StatusGator, which monitors service status pages, has been running profitably for the last year. It took about 3 months to become profitable and it's only so because my time is "free". (But isn't that always the case?)

I don't do anything to promote it so growth has been entirely organic. I get a few free sign ups every day. Although they very infrequently convert to paid, I keep expenses low so it doesn't take many paid users to be profitable. If I had more knowledge of how to market such a thing, I believe it could be a reliable revenue source. But I'm otherwise happy that its relative profitability motivates me to maintain it, as I find it supremely useful for my own personal use.