Here are some other XHTML/CSS related sites I find useful. If you're looking for a good reference, tutorial, or example, try these...

Development News & Blogs

The Daily Report
Much-needed standards-based advocacy from Zeldman, with many interesting links.
A List Apart
Magazine for people who make websites.
A community of website designers, who have written many interesting articles.
Simon Willison
An interesting blog on all things Web-related.
Dave Shea's Mezzoblue
Dave Shea's weblog; the creator of the CSS Zen Garden.

Resource / Tutorial Sites

CSS Zen Garden
Not really a reference, but it will show you of the power of CSS by blowing your mind.
NYPL Style Guide
Harness the power of XHTML with these simple guidelines.
Eric Meyer
Load Mozilla, Opera or Safari and check out his CSS/Edge experiments...
NC Design
An HTML and CSS reference with clear examples, compatibility and syntax. This is where I head first when I'm on the hunt for tag and class info...
REALLY comprehensive set of references on CSS/XHTML/DOM/etc. Bookmark this one.
Index DOT Html/Css
Another great HTML/CSS reference site.
The Nemesis Project
A Wiki-like collection of CSS resources and tutorials/tricks.
Dithered CSS Hacks
Want to fix an browser-specific CSS bug? This is it. The rest of the site's good too.
A good list of CSS layout resources.
QuirksMode (PPK's Site)
Lots of DOM/CSS compatibility resources and JavaScript information!
Explorer Bugs
Find out exactly how poor the IE6 rendering engine really is, and how to fix it.
IE6 Flicker @ FiveSevenSix
Explanation of a problem common to many CSS based sites.

The Horses' Mouths

MSDN Library
Microsoft's spin on DHTML, CSS and JS, well worth a browse. Look under 'Web Development' > 'Web Page Development', as well as 'Scripting'... this site is packed with information.
Netscape DevEdge
Netscape's developer site, has good Javascript and DHTML documentation.
The Mozilla Project: Web Developers
One of the most standards-compliant rendering engines available.
Opera Specifications
Features supported by the Opera browser.

Send Me More!

I'm always looking for useful web development resources to link. So please empty your bookmark folder in my general direction!