cc.cc, co.cc, cu.cc
They are not official hierarchies; they are domains owned by companies who offers free registration of subdomains.
 com.cc, net.cc, edu.cc, org.cc
There are also these second-level domain names maintained by CoCCA. They removed it from their website, but it is still possible to register third-level domain names.
The CO.CC 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 have been used by spammers to create spam blogs, or "splogs," often with nonsense names like "extensitiesdiminutal.co.cc" and the like but all registrants are in fact subject to a terms of service prohibiting standard illegalities.
Step-by-step instructions for registrants on how to use well known hosting services like Blogger, Windows Live, and Google Apps are included on its homepage.
Due to previously large use by website spammers of sub-domains from co.cc, in July 2011 Google removed over 11 million .co.cc websites from its search results. Google's JohnMu recommended that individuals who have legitimate sites (in line with Google's Webmaster Guidelines) on the .co.cc subdomain send a reconsideration request to Google to have their specific site excluded from the ban.
The abundance of cheap .co.cc domains has also been used by those who sell fake "anti-virus" programs.