Mottokrosh's Eldritch Online Character Sheet
This web app is an online character sheet for Pathfinder, with auto-saving, spell-lookup, and more, in a pretty package. It's one my earliest publicly released RPG apps, and it is enjoyed to this day by thousands—I'm not exaggerating—of Pathfinder gamers. The document database is bursting at the seams.
This was written in AngularJS 1, which was all the rage at the time, and has a slim backend in NodeJS, which largely just deals with OAuth authentication (via Google and GitHub), and database (MongoDB) matters.