Linux Documentation en.tldp.org
Linux Man Pages www.linuxmanpages.com
Linux commands directory www.oreillynet.com/linux/cmd
Linux doc man howtos linux.die.net
FreeBSD Handbook www.freebsd.org/handbook
FreeBSD Man Pages www.freebsd.org/cgi/man.cgi
FreeBSD user wiki www.freebsdwiki.net
Solaris Man Pages docs.sun.com/app/docs/coll/40.10
Other Unix/Linux references
Rosetta Stone for Unix bhami.com/rosetta.html (a Unix command translator)
Unix guide cross reference unixguide.net/unixguide.shtml
Linux commands line list www.linuxcmd.org
Short Linux reference www.pixelbeat.org/cmdline.html
Little command line goodies www.shell-fu.org