This is not a complete resource, this is a fast resource. These are the sites that I refer to first, and that I tell people to read. When you want more, just about all of them have their own links to good sites.

Basics
Source
Communicate
Regular Reads
Browsers

Our browsers suck. Just thought I'd mention that. If it was only a question of learning CSS and applying it, there'd be no reason to panic. But years of lousy standards support by the browser companies has made CSS a study in heartbreak. Most of your time will be spent trying to solve browser trouble. This section gets it's own page.

Layout Boxes

This has perhaps the steepest learning curve for people converting to CSS.

Validate
Text Size

Setting text size smaller than default can cause nasty things to happen in IE PC's resizing method. Current wisdom is use ems and refer to keyword 'smaller' in the base. Then Opera comes out too big. Right now I'm using ems and a base size in %. Main text is set as 1.0em, and comes out reduced because it refers to that base %. The upside is: % seems to be stable across browsers, when I want to tweak things I can just change the %, and if some trouble comes up later I can just remove one line and the site will be in a safe default size till I have time to experiment more. The downside is I haven't found the downside yet and that makes me nervous.

Accessibility

Access is not about adding wheelchair ramps to existing pages. It's about getting your page right in the first place. This medium was designed to be accessible. If your work isn't accessible, you're doing it wrong. (This section needs work, but it's a start.)

Media Types

Why are you just writing for the screen? CSS finally opens the medium up to its multi-format promise and you're only writing for screen? What's up with that?

There seems to be little developer-targeted info on the using the other media types yet. This section needs work and is here now simply to remind me to get on it.