These price comparison web sites tend to specialize in computer/electronic related merchandise:
PriceGrabber.com
Shopper.com
PriceWatch.com tends to be the AnandTechers favorite but I prefer the format of PriceGrabber.com
Bear in mind that most of these web sites do not factor in current sales, promotions, coupons, free shipping, etc. Like if you wanted to buy a digital camera you're best of finding the lowest price then buying it from Staples.com instead of that lowest price vendor because:
1) Staples.com has coupons coupons coupons
2) Staples.com will price match a competitor
3) Staples.com is offering a free Iomega 100MB USB Zip drive with the purchase of a digital camera
4) If you order through an ecentives web site you can save 4% off your order.
5) Free shipping over $50.
So even though the price comparison web site says whatever.com has the lowest price it doesn't mean you can't find it for even lower elsewhere.