Phantomjs
Scriptable Headless WebKit
Fastdom
Eliminates layout thrashing by batching DOM read/write operations
Wow
Reveal CSS animation as you scroll down a page
Veinjs
Injects CSS into the document (it's a perfectly legal procedure)
jQuery Evergreen
Small & fast DOM and event library for modern browsers.
Responsive Comments
A client-side solution to conditional loading.
Bespoke.js
DIY Presentation Micro-Framework
Bonzo
library agnostic, extensible DOM utility
Porthole
A proxy to safely communicate to cross-domain iframes in javascript
Persist State
jQuery Ui widget that will transparently save the state of checkboxes, inputs, selects, etc in localStorage, restoring these on the user's next visit to the page.
Voyeur.js
Voyeur is a tiny (1.2kb) Javascript library that lets you traverse and manipulate the DOM the way it should have been.
Html
A simple, extensible way to work directly with the DOM.
Double
jQuery plugin for hiring doubles
Laconic
Sane DOM Generation
Domready
let's you know when the dom is ready
Is Near
Calculates if the mouse position is near to a given element.
Scrob Master
Master your scroll events.