Felony is a top down, open world exploration game inspired by the original GTA written in Javascript using Three.js and Box2D. I'm tracking the progress of development here: http://felonygame.tumblr.com/
The game source is built using grunt, so you'll need to do something like this to get going:
npm install -g grunt-cli
npm install
grunt build
Shoot me a tweet/email first if you'd like to contribute anything to this project - the roadmap is long and not currently public.