This project originally lived on Google Code, and for the time being, the downloads still reside there: https://code.google.com/p/h5o/downloads/list
The extension is available on Chrome Store: https://chrome.google.com/webstore/detail/html5-outliner/afoibpobokebhgfnknfndkgemglggomo
Note: building probably doesn't work and requires some crazy setup while the transition of repos is in progress.