Have found some really good Ajax and effects libraries which can be useful to web developers in their development process.

Lightbox - A simple script to overlay images and create a nice tunning effect to display your pictures on your website.
Link: http://www.huddletogether.com/projects/lightbox/

Thinwire - Framework to build your applications on.
Link: http://www.thinwire.com/

Macao - The framework to build animated and interactive web pages. Uses javascript and DHTML
Link: http://macao.sourceforge.net/

Prototype - A class driven Javascript framework for development of dynamic web applications.
Link: http://www.prototypejs.org/

Script.aculo.us - Effects library, web 2.0 javascript. Has drag and drop and slider controls that anyone can use with ease.
Link: http://script.aculo.us/

Some more reviews I will write once I am finished playing with Yahoo and Google libraries.

I did used http://wildbit.com/demos/modalbox/ Modalbox in past. It allows us to create popups and wizards. Can handle forms too.