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.
These details are provided for information only. No information here is legal advice and should not be used as such.