In its quest to tailor search results to the searcher's exact taste, Google has just introduced Personal Blocklist, a Chrome extension that blocks certain domains or extensions from ending up in your search results. You can set your own preferences, and a link at the bottom of the search results provides easy access to blocked results — just in case you want to check them out.
When users block a site, the information is transferred to Google who can use it to track spam or link-farm sites that can cloud search results. It's like Google is crowd-sourcing parts of its search results cleanup, which probably isn't that bad an idea.
The extension is currently available only for Chrome users; you can download it here.