Februs:
it's been estimated that less than 0.01% of the co.cc sites might have malicious content.
Here's what wiki has to say:
http://en.wikipedia.org/wiki/.cc#co.cc"co.cc" is not an official hierarchy; it is a domain (co.cc) owned by a company who offers free subdomain redirection services.
This company offers two free co.cc subdomains, as well as bulk discounts for ordering as many as 15,000 domain names at a time. These domains are often used by spammers to create spam blogs, or "splogs," often with nonsense names like "extensitiesdiminutal.co.cc" and the like.
Suggestions the co.cc site pops up for free domain names to register include "bestantivir" and "autoloan," belying its true use - the creation of spam domains. Step-by-step instructions for spammers to use free hosting services like Blogger, Windows Live, and Google Apps for Nonprofits are included on its homepage, under the heading "Mapping with co.cc".
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Which has me shaking my head - Google Apps is a spammer service? Blogger and Windows Live don't have legitimate uses?
That said, Google has apparently said that they might relist certain co.cc sites. See
http://searchengineland.com/google-delists-all-co-cc-domains-from-index-83931 for more details.