PicLens

There are loads of useful plug-ins for Firefox but my personal favourite, simply as something that make you go "wow", is PicLens. This plug-in is designed for viewing web images, and turns image views into a 3-D wonderwall. It's like having banks of TV screens each showing a thumbnail of an image, and as you pan left and right through those images the wall accelerates and banks to give a real dizzying impression of speed and movement. You can "flick" the images left and right a-la-iPod Touch/iPhone and see them smoothly accelerate and decelerate. Clicking a thumbnail makes it a bit larger, double clicking makes it full screen and clicking a screen icon takes you to the web page that image comes from. Using the scroll-wheel on your mouse zooms the whole wall in and out.

Use it with a large image set and you get a wall that seems to fade into infinity.

Piclens only really works with sites for which it has been enabled - amongst these are Google image searches, Picasa web albums and Facebook albums.
I love it, it's my favourite way to view images no and I wish it could be adapted as a way to look at images on disk. Maybe they'll come up with that at a later date.