yepnope.js : A Conditional Loader For Your Polyfills!
Sammy.js : Sammy is a tiny javascript framework built on top of jQuery, It's RESTful Evented Javascript.
DOMAssistant : The modular lightweight JavaScript library, with CSS selectors, event handling, AJAX and complete Unicode support
Parsley.js : Javascript forms validation.
Powerful, UX aware & Dead simple.
The HTML5 Shiv : The HTML5 Shiv enables use of HTML5 sectioning elements in legacy Internet Explorer and provides basic HTML5 styling for Internet Explorer 6-9, Safari...
Peer-to-peer data in the web browser.
PeerJS completes WebRTC as an API abstraction, connection broker, and binary serialization format