FreeFocusGames
I’m building https://www.freefocusgames.com, a free collection of browser-based attention and working-memory exercises, including Dual N-Back, Stroop, Schulte tables, attention-control exercises, and memory games. I started it because I wanted simple brain-training exercises that didn’t require an account, app install, or subscription. It’s also open source: https://github.com/loethen/freefocusgames