Just Debugging & Performance Web Tool

Debugging Tools:

1. Firebugs
2. Firepath for selenium
https://addons.mozilla.org/eN-US/firefox/addon/firepath/
3. Selenium tool


PERFORMANCE TOOL:-

Like/Alternative Yslow.
http://gtmetrix.com

More of a network / file size analysis.
http://www.webpagetest.org

http://tools.pingdom.com/fpt/
Very similar to YSlow and Pingtom Tools



Does a side by side race of two URLs
http://www.whichloadsfaster.com/ –

results are generally public but you can download and run this program on your own server if you want. Very useful if you have lots of websites to check and want to automate that process.
http://www.showslow.com/

Yslow.

Going beyond just page load time:
HTML Validator:
HTML validation should be a pre-commit step.
I generally use the HTML Validator plugin in Firefox.
online version is here: http://validator.w3.org/