The Linux documentation project: all docs, manpages, HOWTOs, FAQs
LinuxQuestions.org: forum, downloads, docs and much more
Google for Linux: the specialized search engine
Google Groups: an archive of all newsgroup postings, including the comp.os.linux hierarchy
Slashdot: daily news
http://www.oreilly.com: books on Linux System and Network administration, Perl, Java, ...
POSIX: the standard
Linux HQ: Maintains a complete database of source, patches and documentation for various versions of the Linux kernel.
AlphaLinux: Linux on Alpha architecture (e.g. Digital Workstation)
Linux-MIPS: Linux on MIPS (e.g. SGI Indy)
Linux on the Road: Specific guidelines for installing and running Linux on laptops, PDAs, mobile phones and so on. Configuration files for various models.
MkLinux: Linux on Apple
The Fedora Project: RedHat-sponsored community effort OS.
LinuxISO.org: CD images for all distributions.
Knoppix: distribution that runs from a CD, you don't need to install anything for this one.
DistroWatch.com: find a Linux that goes with your style.
...
Freshmeat: new software, software archives
OpenSSH: Secure SHell site
OpenOffice: MS compatible Office Suite
KDE: K Desktop site
GNU: GNU and GNU software
Gnome: The official Gnome site
RPM Find: all RPM packages
Samba: MS Windows file and print services
Home of the OpenLDAP Project: OpenLDAP server/clients/utilities, FAQ and other documentation.
Sendmail Homepage: A thorough technical discussion of Sendmail features, includes configuration examples.
Netfilter: contains assorted information about iptables: HOWTO, FAQ, guides, ...
Official GIMP website: All information about the GNU Image Manipulation Program.
SourceForge.net: Open SOurce software development site.