I have been using the following Web Development tools over the past few years and they have helped me to be more productive -I discovered 2 new tools that not just tell you what is inhibiting performance of a page but also cut the fat off web pages making them speedier - Doloto & Smush.it
Doloto is a JavaScript download optimizer developed by Ben Livshits of Microsoft Research.
Smush.it is an image optimizer. It reports how many bytes would be saved by optimizing the page's images and provides a downloadable zip file with the minimized "lossless" image files.
Are there any other indispensable Web Developement tools that I'm missing?