A Readium Chrome Extension is also available and can be installed from the Chrome Web Store. Presently (as of Sep. 2013) it has the most EPUB 3 features supported, but is based on an older version of the Readium codebase. In the future, this Chrome Extension will be updated to use the new Readium.js and common JS modules; until then, it is not recommended that developers fork this codebase. A somewhat out of date home page for Readium Chrome Extension, with links to source code, is here.
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),