Hey! This looks pretty handy. The Open Educational Resources Center for California has a pretty unusual URL — http://grou.ps/oercenter/ — but a nice collection of resources that goes into open source education and a little beyond.
The front of this site has a great left nav that leads you to seven different resources for finding open textbooks, four each for open educational resources and open courseware, five resources for open media, and two resources for open quizzes. The front page also has some information on the open education movement and links to additional resources. Looking for something more specific? You can get category links to over 400 open textbooks here.
While you’re at the site, check out the Five Steps to Open Textbook Adoption and the goals of the OER Center for California. It’s a little sad; there’s a forum here but nobody’s participating in it, and the site itself feels a bit empty. Maybe take the ebook resources and turn them into a custom search engine?