QuirksMode.org is the personal and professional site of Peter-Paul Koch, freelance web developer in Amsterdam, the Netherlands. It contains more than 150 pages with CSS and JavaScript tips and tricks, and is one of the best sources on the WWW for studying and defeating browser incompatibilities.
It is free of charge and ads, and largely free of copyrights.

I've been living on this site today, trying to write some complex cross-browser event handlers for javascript.