An author addendum is a proposed modification to a publisher's standard copyright transfer agreement. If accepted, it would allow the author to retain key rights, especially the right to authorize OA. The purpose is to help authors who are uncomfortable negotiating contract terms with publishers or who are unfamiliar with copyright law and don't know the best terms for a modification to support OA. Because an addendum is merely a proposed contract modification, a publisher may accept or reject it.
The list launches with 15 different addenda, but there are undoubtedly more out there. Remember that OAD is a wiki, and counts on users to keep its lists comprehensive, accurate, and up to date.
The open access movement:
Putting peer-reviewed scientific and scholarly literature
on the internet. Making it available free of charge and
free of most copyright and licensing restrictions.
Removing the barriers to serious research.
I recommend the OA tracking project (OATP) as the best way to stay on top of new OA developments. You can read the OATP feed on a blog-like web page or subscribe to it by RSS, email, or Twitter. You can also help build the feed by tagging new developments you encounter.