Parallax has actually been around for a while, but it had its big day in the sun about a year ago when Nike used it to for the wildly successful Nike Better World campaign.
The Nike Better World site is no longer up, but fortunately, there are tons of resources to help you get some Parallax going on your site.
1. Parallax Plugin, by Ian Lunn
2. Build a parallax scrolling website interface with jQuery and CSS, by Jonathan Nicol
3. Scrollorama, by John Polacek
4. Behind The Scenes Of Nike Better World, by Richard Shepherd