This February, I added a premium subscription to Pomofocus and which have generated over $100 so far.
I'm surprised to learn that such a simple tool can actually be profitable!
It's just a web app and not on iOS nor Google Play. The daily unique visitor is around 2,000 and the main customer acquisition is from Google organic search.
Although the money it make is not that significant, this is a great success for me. Because I originally built the tool for myself (I'm still one of the heavy user) and which is also helping other people.