Nowhere near a complete list, but some good links to start seeing what is possible and available without re-creating the wheel:
ThickBox 3.1 built on top of JQUERY
http://jquery.com/demo/thickbox/
Open,Commercial and Reseller Versions library Used by Some Big Folks:
http://extjs.com/
Nice listing of useful scripts:
http://www.howtocreate.co.uk/jslibs/
Two of my favorite sites that have been around for years :
http://www.hotscripts.com/