A Readium Chrome Extension is also available and can be installed from the Chrome Web Store. It is one of the most full-featured EPUB 3 Reading Systems available. The curent version, released September 11, 2014, is 2.15.2.
The Readium project is also developing a cross-browser (as opposed to a Chrome Extension) implementation based on most of the same sources (but with a different viewer/application front end). This is currently functional but incomplete.
All of the ReadiumJS sub-projects are available under the BSD open source license (Readium SDK components, other than the common JS modules shared with ReadiumJS, are available under a dual license model),