Firefox + a few extensions provide a versatile web development environment.
https://addons.mozilla.org/en-US/firefox/addon/2104
Pops up a simple CSS property viewer as you mouse over elements on a webpage.
http://chrispederick.com/work/web-developer/
Provides easy access to web page source, style and scripts. Includes functions such as per pixle window resize, interctive editing and much much more.