That functionality will now be available without an extension (provided you're using versions Chrome 9, IE8 or Firefox 3.5 or higher) as Google is adding a new option to its search that will give you the ability to block domains from appearing in your future search results.
You won't initially see the option to block domains until you've clicked through a link in your search results and then hit the "back" button in your browser. Then, right next to the link to "Cached" and "Similar," you'll find option to "Block all" results from a domain.
Once you've blacklisted a domain, you won't see it in search results, although if it would appear there, you'll get a message telling you that some results have been blocked.
These domains will be connected to your Google Account, so you'll have the ability to see your list of blocked sites and manage them via your Search Settings.
Google says it hopes this will improve search results, and it's bound to if you're irritated enough by certain domains to take the time to block them. Google also says that, at this time, it's not using any information about which sites people choose to block as a signal for search ranking. However Google plans to keep an eye on this data as it looks to improve search results in the future.