Search engines are one of the first places to research products before buying them online, but which one sends the most people to shopping sites? No surprise.

The Hitwise research service reports that Google was responsible for nearly 15 percent of all U.S. "upstream" visits to shopping and classifieds sites last week. An upstream site is the Web page visited immediately before the target site. In other words, 15 percent of shopping-related traffic online came straight from Google.

That makes Google hugely valuable to advertisers, especially compared to rival sites. Yahoo! Search accounted for 4.7 percent of upstream visits, and MSN Search was responsible for 2.3 percent.

Social networking site MySpace accounted for 2.5 percent of upstream visits, which is interesting because you can't really research products on MySpace (not yet, at least, but MySpace has just partnered with Google to incorporate Web search).

By the way, the top three product search terms for shopping last week were Barbie, iPod and Heelys roller shoes.