CentOS 5: Linux for Grownups (Enterprise Networking Planet)
CentOS is more than RHEL with the trademarks removed, which in itself is a big job as you'll see in the Release Notes. (The CentOS team are so paranoid about infringing on Red Hat's trademarks that you'll find hardly any mentions of "Red Hat" in the CentOS distribution or on the Web site. Instead, they refer to it as "UOP", or Upstream Operating system Provider.) They maintain their own package repositories, and apply security patches as they receive them from upstream. CentOS supports a range of hardware architectures as this matrix shows. They're always going to be behind RHEL; with security fixes they're right on top of things, and with things like new releases and support for multiple architectures, they sometimes lag a few weeks behind RHEL. It's free and it's binary-compatible with RHEL, so no complaining allowed."