Epubjs: a browser-based ePub reader implemented in JavaScript
blog.threepress.orgThe reading experience is very smooth. Once mobile devices will be supported (I tried it on my iPhone despite the supported browsers list not mentioning Mobile Safari, but it's not usable there), this might be a viable alternative to native eBook software.
Great job.
She refs two newer ones:
rePublish (LGPL3): http://github.com/blaine/rePublish
Monocle (MIT): http://github.com/joseph/Monocle
I liked Monocle's looks a little better.
16 months old and and she does not plan on updating it. She even links to a couple more advanced JS epub readers in a recent comment.