Window Resizer tests your browser on different screens
Window Resizer (Chrome | Firefox) is a nifty extension for Chrome and Firefox that resize your browser to fit the most common screen resolutions. If you are not sticking to 960 pixels width, then make sure you don't leave your 1024x768 users out in the cold. Another Chrome extension called Resolution Test (Chrome) lets you check multiple resolutions at once.
Dust-Me Selectors finds unused CSS selectors
When your CSS files start growing in size, Dust-Me Selectors (Firefox) will help you reduce their size by pointing out the unused styles. Google Page Speed also does this and points out inefficient selectors as well. Please note this extension will not compress and minify CSS for you (CSScompressor.com does).
Launchlist makes sure you didn't forget anything
Launchlist.net is a great tool to verify if you left any detail behind before going live. The extensive checklist contains everything from design to SEO and has built-in sharing and reporting tools.
Pingdom checks your site's speed
Page speed is essential to keep your customers interested in your website. Anything delaying their access gives them a reason to buy elsewhere. Pingdom's speed test will check your site's speed and return detailed results.
YSlow for Firebug help you make your site faster
This Firebug extension (Firefox+Firebug) will pit your page against Yahoo's best practices for website performance. What's especially great about this tool is that it will explain each test and highlight ways to decrease the time it takes to load your website.
Browsershots captures your site in every possible browsers
If you are willing to wait in line, Browsershots.org is a fantastic browser testing tool. The site offers a wide selection of browsers across every platform, making browser testing easy (although painfully slow). If you own Adobe CS5, you also have access to the much faster Adobe BrowserLab.