Marijn Haverbeke has put together an e-book named Eloquent JavaScript that not only has great content, but also comes with an integrated interface for editing and running the examples directly from the "pages."
I had to post this since the site is such an excellent resource for JavaScript. Running the examples is incredibly intuitive and easy. Excellent work on Mr. Haverbeke's part!
The post/link originated on Ajaxian: http://ajaxian.com/archives/eloquent-javascript