Vendors: Naturally, every good comparison engine needs to establish relationships with as many reputable vendors as possible. Giants like PriceGrabber, Shopzilla, Shopping.com, and Yahoo Shopping generally meet this requirement.

Coupons: Sites like RetailMeNot and MyCoupons.com are great, but that data should not be singled out in a standalone site. It should be integrated into standard search results (like Yahoo Shopping).

Price Alerts: Obviously, if I am truly searching for the best deal, I want to be informed when it happens. The tool to set up price alerts should be prominently displayed. PriceSpider does a good job of this. Sites like Become.com even offer price drops via email without registration. On the other hand, sites like Shopzilla don't even appear to have the feature—or they hide it behind a registration form. Of course, there are numerous sites out there like ZooAlerts, PricePinx and Camel Camel Camel that specialize in price alerts, but I don't see much value in those—especially when they are focused entirely on one shopping site like Amazon.

Price Trending: This is one of the new features on the block. Some comparison engines like NexTag and PriceSpider have already done a good job of integrating this feature in with search results, and it could help consumers rate the quality of the current deal by comparing it to prices in the past.

A Clean, Usable UI: With so many details to keep track of, it's not easy to keep things clean. Personally, I have never been a fan of NexTag's layout—it seems kind of text heavy and convoluted to me. On the other hand Shopzilla has taken a more Web 2.0 approach while Google Product Search stays true to the Google design mantra. In the end, this is really a matter of preference.

User and Expert Reviews: Another no-brainer. Again, these should be prominently displayed with the product.

The bottom line is that on their own, the tools listed above give you only part of the picture—but when used together they can be extremely valuable to consumers.