Search Bleach is a browser extension that removes unwanted domains from
Google search results. Safari and Chrome are supported at this time.

The motivation for this extension is the current fight between Google
and content farms such as eFreedom. These sites have taken copies of
Stack Overflow's user-contributed content and are republishing it with
a mess of ads. These sites are sometimes ranked higher in search results
than the original Stack Overflow page, yet they lag the real content at
Stack Overflow and are generally of much lower quality.

A Firefox extension is planned for the near future.

You can install the extension from Light Year Software's page for Search
Bleach at http://lightyearsoftware.com/open-source/search-bleach.