Smashing Magazine does a great job at identifying several best practice sites and identifying 10 principals to consider when designing a site. Many e-commerce sites make a mistake of an under-designed or over-designed search capability. If the site’s core value proposition is about search (ie shopwiki, google products, etc.) then focus on the user search workflow is often conducted very early in the site evolution. It is not uncommon, however for that differentiator to overwhelm the usability of the site (too many facets, too much granularity, etc.). On the other hand, many e-commerce sites simply have a search box with no ability to filter which puts the burden on the user to sort through results. The only “right” answer is fervent user testing and analysis.