Unix Mages
UnixMages.com is a clever spin on learning how to function at the command line, told as if you were an apprenticed mage learning the "spells" that make the various *nix operating systems bow to your will. Some of the best stuff from the ongoing resource is compiled into a free PDF or alternatively available in bound format for a fee. While the style is slightly unconventional, something about the methodology lends itself to learning the basics of using Linux/Unix/BSD, without the dry nature of typical offerings on the subject.