The OPDS Catalog 1.0 specification has been released.
Here's an excerpt from the announcement:
The open ebook community and the Internet Archive are pleased to announce the release of the first production version of the Open Publication Distribution System (OPDS) Catalog format for digital content. OPDS Catalogs are an open standard designed to enable the discovery of digital content from any location, on any device, and for any application. . . .
Based on the widely implemented Atom Syndication Format, OPDS Catalogs have been developed since 2009 by a group of ebook developers, publishers, librarians, and booksellers interested in providing a lightweight, simple, and easy to use format for developing catalogs of digital books, magazines, and other content.