Simple open source games that are nice and playable.

Monk Story

DotA, video, play, sources, JavaScript & CraftyJS.

Eat them all

Tower Defense / RTS, play (it's not available online, you need to download and run it locally), video, sources, JavaScript & CraftyJS.

Browser Quest

MMO RPG, play, video, sources, JavaScript.

Fantasy Map Tactics

Simple TBS prototype, play, sources, JavaScript.

HTML5 Tower Game Starter Kit

Tower Defense, play, video, site, tutorial, sources, JavaScript.

Neverwell Moor

RPG, play (the site is currently down, you need to download and start it by yourself), video, sources, JavaScript & MelonJS.

Grits

2D Shooter, play (currently there's some minor error on the site that prevents from playing it, but I played it earlier, it was in working state), video sources, tutorial, JavaScript.

You may also be interested in simple JS toys with short and clean sources that may be useful for learning without taking too much time.

By Alex Craft