If you resize your browser window while you are browsing this website you can see the black polaroids in my header fly and move at different speed on three layers. Pretty cool, huh?
This effect drew much attention in the webdesign community and meanwhile there are some great articles about this effect. Over at thinkvitamin is a detailed explanation of the effect. Paul Annett from clearleft gives an in-depth-view how they created the parallax effect on their silverback teaser page.
A more common explanation gives Chris Coyier over at css-tricks.com. He uses three star layers to make a cool space-effect.