The CSS bug ring
Killer CSS link: Position Is Everything, part of the CSS bug ring. Big John on CSS-Discuss is probably the single most helpful individual I have ever encountered on a mailing list—he explains the most complicated (and simple) of solutions quickly, comprehensibly and in easy to understand terms. Position Is Everything is his collection of common but complex CSS browser bugs, complete with full explanations and any effective workarounds. The site links in to the CSS bug ring, a small ring of five sites providing more of the same. Truly an indespensable resource for anyone with an interest in CSS.