I built a tool for predicting the outcome of matchups in Yahoo Fantasy Hockey (head-to-head category based leagues). I find it helpful for determining what categories to focus on when picking up streamers/free agents. https://fantasyhockey.fly.dev/
Also, I've seen a few budgeting apps on here. I didn't build Budget with Buckets[1], but I do think it's a great YNAB alternative _except_ that there is no mobile app. So I built a web app that can be used on mobile. https://buckets.goatcounter.com/