Streak is a Trello Power-Up that allows you to track habits straight on your Trello board.
You check off that you've completed your habit using the calendar on the Trello card, and then you can see at a glance how you're tracking across all your habits with the little badges on the front of the cards.
This is my first Trello Power-Up (and first proper side project, too). It's being hosted for free on Github Pages. You can check it out via Trello's Power-Ups directory.
I haven't been doing any marketing for this, but I've still managed to get over a 100 daily active users organically. It's currently a featured power-up on Trello, so I'm hopefully looking forward to seeing a small boost in users from that.