Honestly, US is such a big market but all price comparison sites here are plain junk, totally useless. Take a look at prisjakt.nu THE BEST in the business (Swedish website, useless for US prices but great if you want to buy a specific product and you can not figure out specifications, like laptops, TVs etc.)
To your question, best is froogle (now google shopping) but be warned of taxes and shipping. Sometimes they fail there.
Or the trick that I use (that usually does not need comparison shopping), rarely someone sells cheaper than Amazon so what I do is, I keep looking for products that I would like to buy sometimes during my free time and I have camelcamelcamel extension installed in Chrome. If I like the product, you click on the icon and it tells you what is the cheapest price amazon has sold the product. I set up a price alert around that price and as soon I get an email that it has met the criteria, I buy the product. So far failed only once.