Amazon is a virtual store with infinite shelf space. Its sheer size makes it the default market maker for just about every product sold online, and thus it sets the prices to beat. Text a UPC code (that string of numbers next to the bar code) to "AMAZON" and you can even check a price when you're out in the real world and think you've spotted a bargain. But don't stop there: thousands of competitors and myriad price-comparison engines allow you to shop them all at once. Shopzilla is one of the best, as it can factor in tax and shipping costs for a true out-the-door price.