Arnaud Leyder
Magneticmediajs is a library that allows media content (images, videos, maps, HTML & more) to be displayed efficiently and with style
W2UI: New JavaScript UI Library
Vitali Malinouski
Complete set of JavaScript UI widget for development of data-driven web applications. Includes layout, grid, toolbar, sidebar, tabs, forms, fields, popup, overlay
James Ducker
Selectify is a UI overlay for select elements which does not screw up your events.
anseki
The simple jQuery Plugin for customizable overlay which covers a page, elements or iframe-windows that is specified.
Video jQuery Lightbox – Fancy Video LightBox!
VideoLightBox.com
Video LightBox for jQuery is a free extension that helps you easily embed video to website, web page or blog. * Embed flv, mp4, 3gp video files * Add Youtube, Facebook, Google Video, Metacafe, Vimeo, MySpace videos * XHTML compliant * Automatic thumbnail creation * Adding caption
Carlos Bonetti
Easily add and manipulate loading states of any element on the page