ModalBox is a JavaScript library for creating modal dialogues without using conventional popups and page reloads. It's inspired by Mac OS X modal dialogues. And yes, it may also be useful for showing bigger versions of images.
Basically, ModalBox is based on GrayBox technique by Amir Salihefendic. But you can find a lot of similar techniques around the web: Lightbox JS, Lightbox gone wild, ThickBox etc.
There also a clone of ModalBox, the MOOdalBox, written on great and lightweight Mootools Framework.