Tools
Below are some useful tools and resources that we recommended to use when building and optimising websites.

Markup Validation Service
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc.

CSS Validation Service
This validator checks the validity of any CSS used in a website.

Keyword Density Checker
Checks the density levels of keywords used in a website allowing you to fix any that aren't used correctly.

Page Rank Checker
Checks a websites Page Rank from all of Google's data centres.

Alexa Traffic Rankings
Shows the top 500 sites that people from around the world are visiting and allows you to search for your site to see where you rank.

Way Back Machine
Take a look at how your website looked in the past - more for fun than anything ;)