For the past few months, I have been slowly developing an Atari 2600 emulator in the form of a web application. The application functionality is written exclusively in Javascript, and takes advantage of several new HTML5 features.
Among the new HTML5 APIs, html5atari will feature Drag and drop, the new gamepad API, web audio, HTML canvas, FileReader, and many more... Check back soon to see updates, or take a fork and help out.