HTML5 Video Player | VideoJS
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
Proudly Announcing: Modernizr | FarukAt.eş
Modernizr is a small JavaScript toolkit that uses feature detection techniques to determine whether or not the current browser is capable of any of the following features from CSS3 and HTML5.