MIT's OpenCourseware site has many classes available to download and adapt via Creative Commons Licensing.
Many of the classes are complete with textbooks, syllabi, exam and quiz questions, and exercises, and some even include recorded lectures.
There are too many to list in this one box, but I've linked to specific areas to help you find what you need.
To download all course materials, make sure you visit the "Download Course Materials" link in the left side navigation.
University of Minnesota's College of Education and Human Development has a catalog of open-access textbooks. Some of the titles have detailed reviews, and faculty from other institutions can volunteer to reveiw textbooks for a small stipend.
Open.Michigan is an online repository developed by the University of Michigan faculty of open access learning materials. There are lots of materials available, and many of the classes have an low-cost purchase option available of some materials for students who want a print copy of readings.
Most of the content on this site is published under CC BY license, which means you are free to use and adapt with attribution.
Literature & the Arts:
The Saylor Foundation hosts nearly 300 full courses in a variety of topics.
The course outlines and syllubi are all CC-licensed by Saylor, so you are free to use, remix, repost, etc.
However, some of the source materials are protected under different licenses, so please check the copyright status before you use it. Saylor.org's website has more detailed information on the copyright status of their materials.
Merlot offers a variety of learning materials: lesson plans, images, videos, syllubi, quizzes and exams, and even textbooks. Items contributed to the Merlot site are peer reviewed by an editorial board.
While you can search all of Merlot, here are some collections that serve as portals to discipline specific materials.
Open Course Library, created with grant funding by Washington State University, provides full courses paired with either free or open source textbooks that are all available via Creative Commons Licensing. Their handy How To Guide provides detailed instructions on how to access and download the class materiels.
For advanced uses, the courses can also be downloaded in Common Cartidge format (compatible with Blackboard) from their Connexions page.
College Open Textbooks is a clearing house site that lists open textbooks by subject. Not all of the textbooks listed have been "vetted" by the site editors, so you should review them carefully before using them in your classrooms.
Smarthistory, a part of Kahn Academy, has multimedia content on art history.
You can search for content by time period, style, artist, and even themes (like oil painting techniques).
Since its founding in 2005, Smarthistory has earned numerous awards, given talks at conferences, and been mentioned in publications all over the nation.
Rice University founded OpenStax press in 2011 to create peer-reviewed, high-quality textbooks under a Creative Commons license. The textbooks also come with other class materials like homework ideas and study guides. They are releasing new subjects each semester, so the collection is slowly growing.
All of these textbooks can also be customized, or used as is.
The Federal Registry for Educational Excellence, or FREE, is curated by the US Department of Education.
While you may not find a textbook in this directory, you will find lots of materials to supplement existing lessons.
Videos, games, primary documents, and images are among the things you'll find recorded in this directory.
