Online and brick-and-mortar Apple retailers sell refurbished or "factory recertified" models that come with the same one year warranty as new machines.
Information regarding testing and getting recertified is also available through their website.
You can also search the site specifically for discounted products, computers with mail-in rebate offers, or recertified products.
The online electronics superstore eCost has great deals in its recertified, clearance, open box, and product bundle sections.
Further, when you are recertified, you are given a score on how good a potential store owner you are.