I'll add:
http://www.alistapart.com
and
http://www.quirksmode.org
which are two of the best sources for compatibility advice between browsers. Also you can validate all your stuff at:
http://validator.w3.org
Also, one of the best web design tools you can get for for is the Firebug add-on for Firefox which is great for debugging (I'm sure some of you are converts already!)
http://www.getfirebug.com/