1. Firebug Lite
Developed by Firebug Team.
2. Speed Tracer
Developed by Google.
Speed Tracer is a tool to help you identify and fix performance problems in your web applications. It visualizes metrics that are taken from low level instrumentation points inside of the browser and analyzes them as your application runs. Speed Tracer is available as a Chrome extension and works on all platforms where extensions are currently supported (Windows and Linux).
3. PageRank Status
Developed by ChromeFans.
PageRank Status is a Chrome SEO extension for webmaster. With this extension, you can view the Google PageRank and Alexa Rank of the current web page, quick access to Whois, Alexa, backlinks and indexed pages in a popup window.
4. Resolution Test
Developed by Ben Beckford.
Resolution Test changes the size of the browser window for developers to preview their websites in different screen resolutions. It includes a list of commonly used resolutions as well as a custom option for you to input your own.
5. W3C (x)HTML Validator
Developed by David Boissonnault.
W3C (x)HTML Validator displays one of 3 icons (valid, invalid, processing) in navigation bar related to validity of (x)HTML document currently shown.
6. SEO Site Tools
Developed by Carter Cole.
SEO Site Tools pulls PageRank, domain links and rank metrics, aggrigates social media reactions from Twitter, Facebook, Digg and SEO releated tags.
7. Domain Availability Checker
Developed by SEOHeap.com.
Domain Availability Checker checks the availability of a domain name. What's the point of going to another page when you can check it from your toolbar.