Post up all the useful sites to test/code etc your websites!
Test your website speeds:
http://www.watchmouse.com/en/checkit.php
some nice patterns for website backgrounds (human brains love patterns, they're far more enjoyable than single color bg's)
http://subtlepatterns.com/
test your websites "durability" / security
http://www.bellum.org.uk/
a ton of file mime types (for php uploads or whatever)
http://www.webmaster-toolkit.com/mime-types.shtml