This is a separate legal document which modifies a publisher's agreement. It spells out the author's rights, allowing them to select which rights they want to maintain for themselves, e.g. the ability to continue to use a copy of the work or the ability for others to use the work.

The following sites will be helpful in determining what rights you want to maintain, how to use an addendum, and offer templates to copy:

http://www.arl.org/sparc/author/
This is from SPARC (Scholarly Publishing & Academic Resources Coalition) with a basic template that can be downloaded.

http://sciencecommons.org/projects/publishing/scae/
This is from the Science Commons and provides a dynamic website that allows you to build an addendum online using their template.

http://libraries.mit.edu/about/scholarly/amendment.pdf
This MIT amendment is well done and gives you good instructions.

http://www.dlib.org/dlib/november06/hirtle/11hirtle.html
This is a very good site from OhioLink that does a comparison of various author addenda.

For an overview of what individual publishers are doing vis a vis accepting amendments to their publication agreements:

http://www.sherpa.ac.uk/romeo.php
Sherpa Romeo is a non-profit organization which tracks scholarly journal publishers.