See for a complete list.

Here are some of the more notable ones:

Texas Hold'em Simulator
Like Flopzilla, except free and not as good, GitHub

CSS Demos
CodePen + video explanation for most of CSS, GitHub

Tater Tot
Watch YouTube videos with your friends, GitHub

Skimmable Videos
Create skimmable YouTube videos, GitHub

Apartment Finder
Use Yelp reviews to find apartments, GitHub

PG Essay Summaries
Summaries of Paul Graham's essays, GitHub