W3C MarkUp Validation Service
W3C CSS Validation Service
I write all my html and style sheets in a text editor, so I like the idea of a place I can go to to check my syntax. (Bear in mind, though, when I was in school I was that goody two-shoes who sat in the front of the class and raised her hand all the time to get the teacher's approval.)
The Layout Reservoir - BlueRobot
BrainJar.com: CSS Positioning
Little Boxes
css/edge
I nipped and tucked from various CSS layouts at all of these sites, but I found those at BlueRobot.com especially helpful.
The image at left was created by Sachiko Kamimura, who did the character design for the anime The Heroic Legend of Arislan. It's from the art book Guardian (ISBN4-257-03498-X).