A List Apart: Great on all kinds of topics, but especially css.

Ask Dr. Web: style, design, css.

CSS Mastergrid: Goes through each different css element and indicates which browsers support what.

Developer's Shed: php, perl, mysql, dhtml, flash, javascript, xml

Glish.com has all kinds of articles on css, dhtml, DOM, XML, etc.

Web-Developer's Virtual Library is super complete and easy to use. Lot's of good information on using specific software tools like Flash or Photoshop

Webmonkey is very reliable and useful. Don't forget to take advantage of its leveled access (beginners, builders, masters).

Training Tools.com has a lot of free, self-guided lessons to using popular tools from Macromedia, Adobe, Microsoft, Miva, etc.

Computer Training 2000 has lots of articles on "How to" do different things with Microsoft apps like Excel, Outlook, and Word.