Michael Sync

LightBox

Lightbox JS is a simple, unobtrusive script used to overlay images on the current page. It’s a snap to setup and works on all modern browsers.

Version 1 ~
http://www.huddletogether.com/projects/lightbox/
Version 2 (Source and Demo) ~
http://www.huddletogether.com/projects/lightbox2/

iBox

Source ~
http://www.ibegin.com/blog/p_ibox.html

Demo ~
http://www.ibegin.com/ibox/ibox-test.html

Thick Box

One box to rule them all..

Source ~
http://codylindley.com/Javascript/257/thickbox-one-box-to-rule-them-all

Demo ~
http://jquery.com/demo/thickbox/

Greybox

Source ~
http://orangoo.com/labs/GreyBox/

Demo ~
http://amix.dk/greybox/demo.html

GreyBox – Redux

Source and Demo ~
http://jquery.com/blog/2006/02/10/greybox-redux/

Yahoo UI Library

The Container family of components is designed to enable developers to create different kinds of content-containing modules on the web. Module and Overlay are the most basic containers, and they can be used directly or extended to build custom containers. Also part of the Container family are four UI controls that extend Module and Overlay: Tooltip, Panel, Dialog, and SimpleDialog.

Source and Demo ~

Yahoo! UI Library: Container
Yahoo! UI Library

UPDATED:ExtJS

Cool and very popular Javascript Library.

Download and Demo : http://extjs.com/
Note: Please let me know if I missed out something. Thanks.