A set of open source libraries for game development: EaselJS is designed to work with HTML5 Canvas, SoundJS is for working with audio, TweenJS is for creating animations, and PreloadJS is for managing the loading of all necessary elements.
An engine for creating 3D games using WebGL. The source code of the project is open and constantly updated.
An engine that allows you to create 2D and 3D games and place ads on those games for profit. PlayCanvas is free for public projects, and you can purchase one of two paid versions with advanced features.
According to the creators, the main advantage of PixiJS is rendering speed. The engine is completely free, it is intended for creating 2D applications: many examples are available on the project website.
There are other useful tools out there, an extensive list of frameworks for developing HTML5 games is available on GitHub. There is even a separate site dedicated to game engines for HTMl5: HTML5gameengine.com.