This is potentially very important for DRM-free ebook sites. It isn't entirely clear what "open" means, but OpenPub
offers the opportunity for DRM-free ebook tools, such as FBReader and Calibre, to have access to free and paid DRM-free content at multiple sites. OpenPub will also deliver DRM-ridden ebooks, but those are not of direct interest of DRM-free tools.
For example, Baen is putting a Stanza catalog in place (Stanza interface for Baen Webscriptions
). It would probably already be possible for another reader app to access this catalog, but if it was an OpenPub catalog how to do so would be documented.