Browser Extensions I Actually Use - The Media Temple Blog
ProtoFluid - HTML5 and CSS3 Web App Prototyping
Test your @media queries and web app on various screen sizes
HTML5 Readiness
Compare browser support of HTML5/CSS3 features
Web Font Specimen
See how type looks on the web
Conditional-CSS
Conditional-CSS allows you to write maintainable CSS with conditional logic to target specific CSS statements at both individual browsers and groups of browsers.