FreeBSD
FreeBSD is a popular free and open source operating system that is based on the Berkeley Software Distribution (BSD) version of the Unix operating system. It runs on processors such as the Pentium that are compatible with Intel's x86 architecture and also on amd64, Alpha/AXP, IA-64, PC-98 and UltraSPARC processors. FreeBSD is an alternative to Linux that will run Linux applications.
FreeBSD is said to be fast, stable, and especially appropriate for use as an Internet server or as a file server. FreeBSD supports all major X Window desktops, such as KDE and GNOME.