Outpost.com scores a decisive victory, helped by free overnight shipping and quick email replies, in the latest PowerRankings of online computing retailers by Forrester Research, Inc. (Nasdaq: FORR). Forrester’s PowerRankings combines survey data from online consumers and unbiased shopping tests to provide objective rankings of the leading US eCommerce sites. The companies that rank below Outpost.com are Amazon.com, CDW.com, buy.com, MicroWarehouse.com. Staples.com, Egghead.com, and TigerDirect.com.

Outpost.com continues to dominate the computing category with its third straight win. The site makes buying easy with optional registration, online order tracking, and quick email support. While the site does not have the lowest product prices, its free standard overnight shipping often makes total order prices lower than most of the other sites tested. But the top-ranked site has flaws, too, including the inability to cancel orders online and the fact that nondefective software cannot be returned.

Second-place Amazon.com is loaded with features like single-click express checkout and the ability to send to multiple addresses in a single order. But product inventory information is vague, privacy policy links are absent from registration and checkout pages, and customer support is frustrating. On average, email replies took more than a day, and three of four phone calls were not answered within five minuets.

An up-and-down experience at CDW lands the site in third. The site provides helpful side-by-side product comparison and the option to phone in credit card numbers, but setting up online order tracking is complicated and there is a 15% restocking fee on returns.

“For the third time in a row, Outpost.com finishes No.1 in computing,” said Tom Rhinelander, research director at Forrester. “And unlike other PowerRankings industries like books and electronics where the competition has heated up, Outpost.com continues to improve its shopping experience, thereby increasing its lead over its rivals.”

buy.com finishes in the top four with features like online order cancellation, clear privacy and contact information, useful online help, and quick email responses. However, its inventory information is vague, there’s no express checkout, and there are no customer reviews or ratings.

For the latest PowerRankings, Forrester surveyed 20,000 consumers from the NPD Group’s online panel. These consumers identified the eCommerce sites that they purchased from most recently and rated their experiences. A team of Forrester shoppers then evaluated the shopping experience on sites that have a statistically valid number of consumer respondents by performing a series of rigorous tests. The consumer data and Forrester shopper scores were then synthesized and weighted, with consumer views accounting for two-thirds of the overall PowerRankings. A complete set of PowerRankings results — both consumer and Forrester shopper data — is made available to all ranked companies free of charge.

Additional information about each of the categories, including a complete set of rankings and scores, can be found can be found at the PowerRankings Web site — powerrankings.forrester.com.