The Scholarly Publishing & Academic Resources Coalition (SPARC) is an alliance of academic and research libraries organized through the Association of Research Libraries (ARL). Their Author Resources page includes the Author Addendum as well as more information about copyright and open access as it relates to academic researchers. See PDFs below!
Another useful addendum tool is the Scholar's Copyright Addendum Engine, created by Creative Commons' Science Commons project. This site allows you to create a PDF personalized with the name of your article and the type of copyright agreement that applies, which differ mostly in terms of the embargo period preferred and ability of others to reuse your work.
The "How Open Is It?" guide from PLOS and SPARC can help answer that question.