From CNET News.com:
The clone is from a project called CentOS--Community Enterprise Operating System--one of several "Red Hat rebuilders" that have partially nullified Red Hat's business decision in 2003 to stop giving away its supported and certified product for free. CentOS and others--Lineox, White Box Linux, Tao Linux, X/OS Linux and Scientific Linux--all rebuild a copy of Red Hat Enterprise Linux from the source code components Red Hat releases.