The Linux Documentation Project
An excellent source of information about Linux
Linux Commands
A summary of Linux commands
Linux.org
The home page of Linux Online
Red Hat Linux
The home page of Red Hat Linux
SCO UNIX
The home page of the SCO Group
FreeBSD UNIX
FreeBSD is an advanced BSD UNIX operating system for the Intel compatible (x86), DEC Alpha, and PC-98 architectures. It is developed and maintained by a large team of individuals.
Additional platforms are in various stages of development.