Is.js
Micro check library
Radio
Dependency-free Chainable Publish/Subscribe Library for Javascript
History.js
History.js gracefully supports the HTML5 History/State APIs (pushState, replaceState, onPopState) in all browsers. Including continued support for data, titles, replaceState. Supports jQuery, MooTools and Prototype. For HTML5 browsers this means that you can modify the URL directly, without needing to use hashes anymore. For HTML4 browsers it will revert back to using the old onhashchange functionality.
Bacon.js
FRP (functional reactive programming) library for Javascript
Vis
Dynamic, browser-based visualization library
Platform.js
A platform detection library that works on nearly all JavaScript platforms.
Csso
CSS-optimizer
Supermodel
Supermodel - Minimal Model Tracking for Backbonejs
Mediator.js
An implementation of the mediator pattern for asynchronous events in Javascript
Rx Js
The Reactive Extensions for JavaScript
Superagent
Ajax with less suck - (and node.js HTTP client to match)
Stampit
Create objects from reusable, composable behaviors.
Sentry
Sentry is a realtime error logging and aggregation platform
Fluxible
A pluggable container for isomorphic flux applications
Steal
Gets JavaScript
Modernizr
Modernizr is a JavaScript library that detects HTML5 and CSS3 features in the user’s browser.
Bottlejs
A powerful, extensible dependency injection micro container for JavaScript applications