Linux

CentOS
CentOS is a 100% compatible rebuild of the RedHat Enterprise, in full compliance with RedHat's redistribution requirements. It is for people who need an enterprise class OS without the cost of certification and support.

Debian
A Linux distribution which is made up of a kernel, basic tools and applications software. Debian is distinctive because it is produced entirely by volunteers who share a commitment to the development and dissemination of free software.

Fedora
Fedora Core is intended to be a logical upgrade path for previous users of Red Hat Linux whose needs are consistent with the objectives of the Fedora Project. It is a return to Red Hat's roots; 2-3 releases per year, and freely-available downloads that can be called by name even when media containing those bits are sold by third parties

Red Hat Enterprise
Red Hat Enterprise Linux ES provides the core operating system and networking infrastructure for a wide range of entry-level and departmental server applications. It is ideally suited for network, file, print, mail, Web, and custom or packaged business applications.

Windows

Windows Server 2003
Windows Server 2003 is the most productive infrastructure platform for powering connected applications, networks, and Web services from the workgroup to the data center. Easy to deploy, manage, and use, Windows Server 2003 helps you build a secure IT infrastructure that provides a powerful application platform for quickly building connected solutions and an information worker infrastructure for enhanced communication and collaboration anytime and anywhere.