Select2 - This is a jQuery-based replacement for select boxes. It supports searching, remote data sets, and infinite scrolling of results. The power of this library comes from the ability to turn your user inputs into components which support tons of user manipulation (like search and multi-select) and make it easy to propagate/retrieve this data to/from server as well render/load results on the page. After having to do a lot of work for data-to-input-to-data implementations in the past, I am pretty excited about trying this library out. Oh yeah, and its inputs play nice with infinite-scroll :)
Three - A lightweight 3D library for “dummies” :) This library provides HTML Canvas, <svg>, CSS3D, and WebGL renderers. It combines a lot of these great, modern rendering implementations into one easy-to-use framework and makes things like rendering a 3D cube simple. Check out all the truly awesome examples here: http://threejs.org/
jQuery - Kind of amazed it is seventh on the list right now. I assume this is because everyone just takes it for granted, but it’s really #1 for me. I’m not going to leave a description for this one, because if you don’t already know what this does, then you need to hit the internet hard immediately!
Bower - A great package management solution which uses npm and Node.js under the hood. It exposes your package dependencies via an API to be consumed by your (and public) tech stacks. Bower is a widely used framework, which allows consumers to assemble packages and install components into their applications from around the web.
…also, since I mentioned this a couple times regarding currently popular MVC frameworks, here are the candidates laid out on the spectrum:
Video.js - easy to use for videos
Jquery UI - widgets built on top of jQuery
Less - programatic CSS
Code Mirror - in-browser code editor
typeahead - type-ahead
Special honorable mention:
RequireJS - I use this library in all of my projects because it makes all of your JS files/components easy to load and manage just like classes are in good back-end object-oriented languages. Just like that one commercial says, “I put that **** on everything!”