Computers and the Internet

CompTIA - CompTIA is the organisation behind the most well known sorts of technology certifications, including "A+" and "Linux+".
Insecure.org - Home of the infamous NMap free port scanning utility, a tool no SysAdmin or hacker should be without.
The Internet Archive - An ambitious project to archive the entire internet for posterity. Really, check out the "Way Back Machine", type out the address of any site that's been online for a while and odds are they've got it (or, more accurately, a previous version of it) archived.
Privacy.net - An excellent source of security information, advice, and paranoia. Note that at the top of the page is information about your computer, click on the "For a full analysis Click Here" link to really have the shit scared out of you.
The Register - Irreverent tech related news site from the UK.
Slashdot - THE internet/tech/geek source for news and discussion.
Webopedia - A great resource for information about web and computer terms.

Computer Construction/Mods

Ars Technica: The PC Enthusiast's Resource
Bling Method
Extreme Tech
PC Stats
Tom's Hardware
Twisted Mods

Apple Computers/Mac OS

Apple - The online home of Apple Computers.
Mr. Barrett - A weblog by an Apple Certified trainer, focuses more on general computer issues and life in general than Apple.
Daring Fireball - This would go in the weblog list except for the fact that it's written like an article. Insightful opinion and analysis about all things Apple related.
Folklore: The Original Macintosh -
MacInTouch - An extremely informative news site that hosts discussions about Apple, Macs, and everything related to them. A great place to go if you're having a problem (odds are they're already discussing it), or just want to check out the latest developments and what other people think of them.
The Mac Observer
Mac Windows
MacMentor - A great site geared to helping Windows users make the switch (or decision to switch) to a Machintosh, and helping new Machintosh users get their barings. The best part of this site is the forums (I post there regularly), where you can ask anything about Macs, Apple, or switching, or just talk about your own experiences.
Think Secret

Linux/Open Source

Almost Free Linux -
FreeOS.com -
Just Linux -
Linux.com -
Linux.org -
LinuxISO.net -
OS News -

Shopping

Case-Mod.com
Directron
Frozen CPU
New Egg
Think Geek

Web Design

About HTML
CSS Zen Garden
HTML 4.0 Reference
HTML Bad Style Page
The World Wide Web Consortium
Web Monkey: The Web Developer's Resource