Go to the Apple Online Store, and look on the left sidebar. Scroll down and you'll see Refurbish Mac.
Refurb Macs are macs that were taken in within a certain about of time (less than 14 days?) and made all nice again by Apple. You may also get a free upgrade or so. The refurbs are cheaper than the regular new Macs.
Edit: Fast replies guys. 😀 Yeah, what they said.