The hackable text editor
We need a better Markdown previewer.
The open-source Trello-like kanban (built with Meteor) https://wekan.github.io
uBlock Origin - An efficient blocker for Chromium and Firefox. Fast and lean.
A Regular expression game for Android
CSS is powerful, you can do a lot of things without JS.
Draws simple SVG sequence diagrams from textual representation of the diagram
Freeciv-web is an Open Source strategy game implemented in HTML5 and WebGL, which can be played online against other players, or in single player mode against AI opponents. https://www.freecivweb.org/
Game for learning how to code. http://codecombat.com