These are some resources that I have found useful ...especially the last one:
http://validator.w3.org
http://jigsaw.w3.org/css-validator/
http://www.digg.com/
http://www.famfamfam.com/
http://www.alistapart.com/
http://www.lipsum.com/
http://www.digimark.com/
http://www.ejeliot.com
http://www.color-wheel-pro.com/color-meaning.html
http://www.useit.com
http://www.webXact.com
http://www.alvit.de/handbook
http://www.istock.com
http://www.contentquality.com
http://www.derekallard.com/