Google Lets Users Blacklist Sites From Search Results
Found on Wired on Wednesday, 09 March 2011

Google is giving users the ability to block sites that annoy them from ever showing up again in their search results, via a new link next to search results.
"In addition, while we're not currently using the domains people block as a signal in ranking, we'll look at the data and see whether it would be useful as we continue to evaluate and improve our search results in the future."
It's surprising that Google hasn't introduced this already years ago. It's a simple, buf efficient concept: let humans sort out the spam. On the other hand, Google most likely makes a nice profit from Adsense on spam sites too.