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.
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.
Looking for some science education sources? My NASA Data is more than just a data repository.
They have full lesson plans on contemporary science topics arranged by grade level. The even have science project ideas.
The site also includes the Live Access Server, which provides real data sets with lesson plans built around them for teachers to use in classrooms.
Data sets can also be downloaded into excel files.
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.
"The Teacher Education Editorial Board designed this site to help instructors understand how to use MERLOT materials in their classes. For general pedagogy resources, visit MERLOT’s Pedagogy Portal."
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.
Much of the material is targeted for a K-12 audience, so education majors should find materials that will work well for creating lesson plans for many different education levels.
Curriki collects K-12 teaching materials on a wide variety of subjects.
Educators from around the world have been contributing content since 2006.
To get the most out of this site, create a free account. The search box is in the upper right hand corner. Type a topic and get started.
Materials available include videos, games, lesson plans, images, exercises, rubrics, full courses and more.
PBS LearningMedia is a collection of videos, lesson plans, interactive media and other learning materials geared toward K-12 educators.
You can search for material by grade, subject, standard and collection.
In order to use this resource, you must create a free account.
Many of the resources can be downloaded from the site, but some have restricted use. Their help pages have more information about restrictions and how to find them.
They also have a professional development section to help you hone your teaching skills.