Cross Platform, Lightweight jQuery Based Huge UI Toolkit: jKit
jKit is an easy to use, cross platform, jQuery based UI toolkit, that’s still small in size, has the features you need, and doesn’t get in your way of doing things!
There are a lot of effects and components, including tooltips, charts, parallax scrolling, and a lightbox. The project has detailed documentation on each of the bundled plugins.
jKit has most of the features you ever need. And even if jKit doesn’t have that one feature you need right now, jKit is fully extendable with plugins and command replacements, all that and your API always stays the same.