From the OPDS dev group blog:
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.
The specification is available at: http://opds-spec.org/specs/opds-catalog-1-0.
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.
So what's the big deal? Well what do you think of the ability to search for ebooks from PG or Feedbooks from inside Aldiko or Stanza? The OPDS standard is what makes it possible.
I know that I've covered it before, but this is important. Connecting an ereader owner with free content adds value to their purchase.