While looking for something else, I stumbled on this great Firefox Greasemonkey script called greased lightbox.
Greased Lightbox is a Greasemonkey/Creammonkey user script designed to enhance browsing on websites that link to images such as Google Image Search, Flickr, Gmail, and Facebook.
It basically allows you to follow a link to a larger image without leaving the page. Great for looking at thumbnail images. It also works with Safari if you have Creammonkey installed...
Technorati Tags: greased lightbox, greasemonkey, flickr