uBlacklist
Blocks specific sites from appearing in Google search results
...You can install uBlacklist from Chrome Web Store, Firefox Add-ons or Mac App Store. To block a site that you are looking at from appearing on the search result page, click the toolbar icon. A "Block this site" dialog will be shown. In recent versions of Chrome, the toolbar icon may be hidden by default. If so, first click the puzzle piece icon. To see and edit blocked sites, open the options page. It can be accessed from the toolbar icon. Blocked sites are displayed on the top of the options page. After editing them, don't forget to press the "Save" button.