Regularly

So I built Regularly (https://regularly.co). It started a few years ago with one game, Countable, which I made for my then girlfriend (now wife) and me to play over morning coffee. One marriage and four games later it's a full daily set: no ads, no sign-up, no IAPs. It spread by word of mouth and a few hundred people now play every day.

The bit we care most about is difficulty. Every puzzle is hand-picked, and we track games started vs games completed each day, then re-tune against that. The target is the zone where you have to think but you aren't stumped. Get that wrong two days running and people quietly stop coming back.