Greased Lightbox is a nifty Greasemonkey (and Creammonkey for Safari) script by Joe Lencioni that I came across yesterday. When it finds links to image files, clicking those links causes the image to pop up with the suddenly-popular "lightbox" effect rather than navigating away from the linking page. Not only is it a cool effect, it makes browsing image galleries and Google Images a lot smoother. It also has extra support for sites like Flickr, Facebook, and Gmail, which is a nice bonus. If I were to add anything to Greased Lightbox it would be some kind of slideshow functionality, but it seems to be under active development so perhaps that's coming.