TopUp is another easy to use Javascript library for unobtrusively displaying images and webpages in popups. The library is jQuery and jQuery UI driven in order to maintain cross-browser compatibility and compactness.

TopUp is absolutely free, also for commercial use. The installation of TopUp consists of including only one file (no images or stylesheets), the look-and-feel is Apple-like, the animations and transitions look fancy, you can still use Prototype, it is so easy to specify options and last but not least: you can stay updated with the latest version without changing any code.

Requirements: jQuery Framework
Demo: http://gettopup.com/
License: Creative Commons License