jquip : Smaller, Lighter, Faster, more modular jQuery - include only the parts you want! Don't use it, Don't include it.
Bliss.js : Want to use Vanilla JS but find native APIs a bit unwieldy? Bliss is for you
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...
Rickshaw : Rickshaw is a JavaScript toolkit for creating interactive time series graphs.
Prototype : Easy Ajax and DOM manipulation for dynamic web applications
Sammy is a tiny javascript framework built on top of jQuery, It's RESTful Evented Javascript.