Ryan Johnson has created Control.Modal — a light weight, unobtrusive JavaScript library for creating modal windows and lightboxes using HTML that is already on the page, images, AJAX calls or iframes. It can be added to your existing code in one line, completely styled with CSS, and supports a wide range of configuration options and callbacks.

Control.Modal weighs in at around 8K and hit a sweet spot for some functionality. This script uses the Prototype javascript framework. You must include version 1.5 in your pages before including this script.

Download | More info on usage and example