TamperData

Oh, I like this – a great tool for any web tester, and a better alternative to using CURL to manipulate POST/GET http data. TamperData is an Firefox extension to track and modify http/https requests. It is ideal to common more lightweight security issues, which are important to consider, given the amount of amateur hackers, and the software available to them. As an aside, if you want a serious stab at (snigger!) penetration testing, try the Backtrack linux distro. Use a few tools on this and, believe me, you will never do anything that requires security, across a wireless network.

Try the CSS Validator on this site

Try the CSS Validator on this site (based on CSSTidy)

CSSTidy is an opensource CSS parser and optimiser. It is available as executeable file (available for Windows, Linux and OSX) which can be controlled per command line and as PHP script (both with almost the same functionality).
In opposite to most other CSS parsers, no regular expressions are used and thus CSSTidy has full CSS2 support and a higher reliability.