It's pretty damn useful. And absolutely free. God, I love open source engineers.
ThickBox is a webpage UI dialog widget written in JavaScript on top of the jQuery library. Its function is to show a single image, multiple images, inline content, iframed content, or content served through AJAX in a hybrid modal.
(If, like me, you have no idea what that means, just click on it and check out the examples. Geekity goodness.)
ThickBox 2.0