D&C GNU/Linux User Group

Books (free)
http://www.ubuntupocketguide.com/index_main.html
Free Ubuntu Pocket guide by Keir Thomas. I assume a significant amount of the content will be relevant to other GNUⁱi/Linux distributions.
http://ubuntu-manual.org/
Ubuntu manual project,
http://ftacademy.org/materials/fsm/2
545 page free Linux administration book. I have not read it, so cannot comment on its quality; but certainly worth a look.
Older Hardware
http://www.linux.com/archive/articles/52134?page=1
Reviews of Debianⁱi, Slackware, Damn Small Linux, Puppy, Vector, and STX running on an older machine. Written by Joe 'Zonker' Brockmeier.
http://www.tuxradar.com/content/resurrect-your-old-pc
A nice article discussing several distributions suitable for older hardware. They chose Vector Linux for their test.
Why GNU/Linux?
http://polishlinux.org/why-linux/
10 good reasons to choose GNU/Linux. Arguably, you could add Community Ethos as the 11th reason to choose GNU/Linux. Site is also useful for other Linux topics.
http://www.getgnulinux.org/
A good looking, simple site, making basic explanations for new users. Apparently has a Fedora/Ubuntu bias.
Window Managers and Desktop Environments
http://xwinman.org/
A very good starting place for learning about the numerous window managers and desktop environments. Thanks to Grant for recommending it.
www.maketecheasier.com
Not all will like this. But if you want to see how far the Linux desktop has evolved and how glitzy you can make it, check out these great looking desktops.
X Server
http://www.cyberciti.biz/faq/running-multiple-x-sessions/
Run multiple X sessions on the same computer. Use ctrl+alt+[F7-F12] to switch between sessions.