This is a collection of web-building resources, focusing on accessibility and CSS-based design.
Excellent slideshow/presentation by Douglas Bowman titled CSS: The Good, the Bad, & the Ugly. It details current and recently past techniques, why they were good attempts, how they fail, and what the future holds. As an introduction to the current state of CSS-based design, it is quick and rigorous, but an excellent overview of an ever-changing technology.
If you're looking for more than the previously mentioned Advanced Tables Tutorial, check out Isolani's Creating Structured Tables. It's a bit more on the practical side.
There is an interesting discussion here about wrapping a graphical logo within an H1 tag (Fahrner Image Replacement). Most seem to agree the semantic meaning is of most importance, but disagree whether or not the logo (company name) is semantically deserving of being the primary heading.
The NILS Australia has released a beta version of their Accessibility Toolbar for Internet Explorer. It is free, licensed under the Creative Commons license.
Not to leave other browsers out, there is the Web Developer Extension for Mozilla and Firebird.
While relying on client-side scripting, there are always bookmarklets for Opera and cross-browser.
The W3C released a working draft yesterday concerning the Inaccessibility of Visually-Oriented Anti-Robot Tests. These tests are employed in a variety of situations, presenting a distorted image of a word in an effort to stop automated form registration used by spammers and service abusers. The problem is that these images make it impossible for people with poor vision to access the service. While Microsoft's Hotmail provides an audio alternative, it is still problematic for people without soundcards, the deaf-blind, and even hearing people.
There is currently no good solution at the registration stage. Calling the verifications "Turing" tests is a complete misnomer when they fail to verify real humans. I'd like to see everyone to be able to access a service, and abusers caught by behavioral heuristics. Preventative measures are nice, but in this case too many people are getting caught on the wrong side of the fence.