Colored Boxes: CSS layout, step-by-step, the how and the why. Fabulous.
Holy CSS, Zeldman! is a pretty comprehensive index of CSS, accessibility, and web standards sites. The author has highlighted the ones he considers essential, and I think the only one I’d call essential that he’s missed is Dive Into Accessibility.
Someday, if y’all keep after me about it, I’ll organize my gigantic bookmark list into something more course-like: not just the essential links, but what order to visit them.