Google's experimental "preferred sites" option allows users to overweight certain web sites in the search engine result pages. It debuted in January but at the time was not available to all users. Now anyone can sign up for the feature via Google Labs.
Once you sign up, Google recommends pages from your history that you tend to visit when searching. You have the option to make these sites (or any other) a preferred destination. Further information is available here as well as on the Google Operating System blog, which first reported on the feature earlier this year.
