Other tools

w3c markup validator - HTML validator, find errors in your code
w3c css validator - CSS validator
Color Schemer - online and offline
robots.txt checker - check if your robots.txt contains errors

Resources

famfamfam.com - free icons
Icon Finder - massive collection of icons

Documentation

php.net - official php documentation
mysql.com - official mysql documentation
1keydata.com - online programming tutorials: HTML, SQL, CSS and PHP