Devdocs.io is all of the API documentation for an expanding (and customizable!) list of web technologies including HTML, CSS, HTTP, JS, and even libraries like jQuery and Sass. Fully keyboard-accessible and incredibly snappy, it’s my new best friend. I hope support for scripting languages is coming soon.
Update: Devdocs.io is on fire, with support now for scripting languages like Ruby, PHP, and Node.js, data stores like PostgreSQL and Redis, and even utilities like Git. All in one place.