#1: Pop
On the heels of its first F8 Developer Conference in three years, Facebook open-sourced this extensible animation engine for iOS and OS X. On top of basic animations, Pop supports spring and decay dynamic animations with an API, allowing quick integration with existing Objective-C codebases, thereby enabling the animation of any property on any object.

#2: Velocity.js
Velocity is a jQuery plug-in for accelerated JavaScript animation that re-implements $.animate() to speed up performance. Created by Julian Shapiro, the animation plug-in is faster than CSS animation libraries while including improved animation workflow features such as color animation, transforms, loops, easings, class animation, and scrolling. Velocity is the best of jQuery, jQuery UI and CSS transitions combined.

#3: Famo.us
Famo.us is a JavaScript framework designed to build rich, interactive Web applications, maintaining tight control of rendering to optimize performance and strict modularity for flexibility. Paired with server-side technology designed to match Famo.us on the front end, developers can create the types of high-quality applications once reserved only for native development.

#4: Isomer
Developed by Jordan Scales, this simple isometric graphics library for HTML5 canvases allows for easy creation of isometric graphics. Isomer is divided into three basic components: points, paths and shapes. It also has 2D paths, 3D modeling to scale, the ability to translate and rotate shapes, and smaller classes to reduce code repetition.

#5: GitHub Cheat Sheet has been a long-time occupant of this list.