Debian GNU/Linux is a free distribution of the GNU/Linux operating system. It is maintained and updated through the work of many userswho volunteer their time and effort.
Super-fast and great-looking, Ubuntu is a secure, intuitive operating system that powers desktops, servers, netbooks and laptops. Ubuntu is, and always will be, absolutely free.
CentOS -- Community ENTerprise Operating System is a free rebuild of source packages freely available from a Prominent North American Enterprise Linux vendor.