Chosen is a JavaScript plugin that makes long, unwieldy select boxes much more user-friendly. It is currently available in both jQuery and Prototype flavors.

jQuery File Upload – jQuery UI version

File Upload widget with multiple file selection, drag&drop support, progress bars and preview images for jQuery. Supports cross-domain, chunked and resumable file uploads and client-side image resizing. Works with any server-side platform (PHP, Python, Ruby on Rails, Java, Node.js, Go etc.) that supports standard HTML form file uploads.

jQuery UI Progressbar

Displays a status indicator for loading state, standard percentage, and other progress indicators.

jQuery UI Droppable

Enables drop targets for draggable elements.

jQuery UI Shake Effect

Shakes an element horizontally or vertically n times.

jQuery UI Button

Enhances a form with themeable buttons.

jQuery UI Puff Effect

Creates a puff effect by scaling the element up and hiding it at the same time.

jQuery UI Slide Effect

Slides an element in and out of the viewport.