A list of great css only designs, all based on the same content:
http://www.csszengarden.com

A list of css resources:
http://www.dezwozhere.com/links.html

A graphical index of css layouts to forget all about tables and start using css:
http://blog.html.it/layoutgala/

A generator to create your own ajax loader icons:
http://www.ajaxload.info/

Javascript prototype framework (and lots of derived frameworks):
http://www.prototypejs.org/

Joomla, an opensource CMS:
http://www.joomla.org
http://www.dutchjoomla.org

Visual index of lots of free Joomla templates:
http://www.joomlaos.de/joomla_template_galerie.html

color combination chooser (colorwheel):
http://www.colorsontheweb.com/colorwheel.asp

Firefox has lots of interesting developer plugins:
http://www.getfirefox.com
http://cappellehans.blogspot.com/2007/06/yes-i-use-firefox-my-plugins.html