I found this java script by accident and thought it looked interesting. It will magnify a thumnail size image to fit the full brouser window without opening a new window. It's free for both personal and commerical use. I haven't had time to play with it yet but am thinking about it for my web site.

http://www.c6software.com/Products/PopBox/

Looks really useful - thanks, I've bookmarked it.