Open Source - Operating Systems
http://fedora.redhat.com/
Fedora Core is a free operating system that offers the best combination of stable and cutting-edge software that exists in the free software world.
http://www.novell.com/
The Platform for the Open Enterprise. SUSE® Linux Enterprise 10 is the only enterprise-grade Linux* platform offering a complete solution for your mission-critical needs—from the desktop to the data center. When security, cost and reliability matter, companies choose SUSE Linux Enterprise from Novell.
http://www.nitix.com/
Net Integration Technologies Inc. is a software developer that delivers autonomic Linux-based server operating system solutions to the SMB market.
http://www.gentoo.org/
Gentoo is a free operating system based on either Linux or FreeBSD that can be automatically optimized and customized for just about any application or need. Extreme configurability, performance and a top-notch user and developer community are all hallmarks of the Gentoo experience.
http://www.mandriva.com/
Mandriva is the company which develops Mandriva Linux: the strong graphical Linux Operating System, famous for being easy to install and ready to use as a desktop or as a server. Mandriva also accompanies its products and first-class consultancy, support and training for Linux.