Over the last year, websites that duplicate the content from other sites ("content farms") have sometimes ranked higher in Google than the site that hosts the original content. This has been a particular problem for Stack Overflow, which makes all of its user-contributed content available in dumps. Sites like eFreedom and ComAnswer republish Stack Overflow content wrapped with ads, but because Stack Overflow is an interactive site, these clones are often out of date and almost never what the user is searching for.
Search Bleach is my answer. On Google search result pages, it simply removes any hits from domains in the block list.