XHTML validator. This site checks documents like HTML and XHTML for conformance to W3C Recommendations and other standards: http://validator.w3.org/
W3Schools--Well organized and easy to understand online Web tutorials based on W3C Web standards: http://www.w3schools.com/
A List Apart has a lot of great CSS tips, tricks, and tutorials... a great resource for web developers: http://www.alistapart.com/
Lay-out-o-matic. Generate standards compliant CSS layouts with the with a few clicks of the mouse: http://www.inknoise.com/experimental/layoutomatic.php
Glish. Another great site that has a ton of CSS techniques, layouts, resources, and tutorials: http://glish.com/css/
CSS Maxdesign. Home of Listomatic and Listomatic2. One list, many options: http://css.maxdesign.com.au/index.htm
Another CSS template creator: http://www.csscreator.com/version2/maketemplate.php
More templates: http://www.mentalized.net/cssdepot/ and http://webhost.bridgew.edu/etribou/layouts/
Devshed forums. A great place to get support from peers and pros... signing-up is easy: http://forums.devshed.com/f116/s