Readium was launched by the IDPF in February of this year. It was originally intended as a proof of concept app which developers could compare their own home-built Epub3 reading apps to, but it has since been adopted by some readers and institutions.
In fact, just a few days ago Benetech announced that their new accessible browser based reading app, which is used by thousands of visually impaired readers, was now based on Readium. That app was modified heavily and tweaked to fill Benetech's needs, and at least some of the improvements which Benetech made will be integrated back into the core Readium app.
You can find the Readium Cloud Reader here. I was just playing with it in Firefox, to be honest it is rather buggy. But it is an interesting option, and it is going to get better.