According to Network World, some 22.4 percent of Google searches performed since June produced maliciousURLs (such as fake antivirus sites or malware downloads) as part of the top 100 search results, as opposed to 13.7 percent in the second half of 2009.
Most reputation services for identifying malicious sites relies on a large install base of deployed appliances that report new URLs and their associated behavior back to a central database for automated inspection backed up by teams of researchers for those sites that defy automated analysis.