jQuery Transit creates super-smooth CSS3 transformations and transitions for jQuery. Use $('...').transition instead of jQuery’s $('...').animate, it has the same syntax as animate.

jQuery Plugin Boilerplate is another boilerplate for jump-starting jQuery plugins development. It contains lots of comments to help you get going easily. It is truly object-oriented, and it implements public and private methods as well as public and private properties, making it the ideal candidate for when building both simple and complex jQuery plugins.

ht5ifv is an extensible (allowing the programmer to define new restrictions beyond those already defined by HTML5) and highly configurable (allows changes to the validation rules defined by HTML5 and the addition of new ones) jQuery plugin which allows validation of the HTML5's forms while the user is typing.

jRumble is a jQuery plugin that rumbles, vibrates, shakes, and rotates any element you choose. It's great to use as a hover effect or a way to direct attention to an element.

pdf.js is an HTML5 experiment that explores building a faithful and efficient PDF renderer without native code assistance.

Supported by Mozilla Labs, its goal is to create a general-purpose, web standards-based platform for parsing and rendering PDFs, and eventually release a PDF reader extension powered by pdf.js. Integration with Firefox is a possibility if the experiment proves successful. – PDF Reader