My friends and I get together every week to play multiplayer games of Magic: The Gathering. We got annoyed at tracking life by hand, especially when people started playing life gain deck. There weren't any great apps that let each person manage their own life total AND let them see everyone else's life. On top of that, we have people using both Android and iOS in our play group.
I used the opportunity to learn React and built a web-based app that lets people:
Create games and invite other people by copying and sharing a simple URL
Track changes to anyone's life total in real time without refreshing
Look up cards (and share them, making them pop up on everyone's screen)
Track commander damage by player (which a lot of other apps do poorly)
Brand, UX Design, Visual Design, React/Redux