Nibbler - testing website

Nibbler is a free tool for testing websites (still in alpha at time of writing). It takes a web address you provide, tries to find up to 5 pages from inside it, and tests them for a range of criteria. At the end you get a free report scoring the website out of 10 for various tests, and awarding a 'Summary score' for all the tests.

CSS3 Sandbox

CSS3 Sandbox - an outstanding tool for using CSS3 features such as rounded corners, linear & radial gradients, drop shadows etc

"Here's a collection of tools for web developers he's put together to help you analyse and debug your sites (and other people's sites as well), play with CSS3 features (bleeding edge browsers recommended) and more..."

Thanks to John at StyleMaster

Highcharts - interactive JS charts for web pages

Highcharts is a JavaScript charting library for creating interactive charts in various types (line, spline, area, areaspline, column, bar, pie & scatter)