Open Educational Resources are freely accessible educational resources that can be used, re-mixed, and altered for educational purposes. They can include full courses, parts of courses, videos, handouts, tutorials, etc...
The Open Education Consortium is a global, non-profit consortium of educational institutions that advocates for global open education. Its website includes news and information on recent initiatives.
Pixabay: "...copyright free images and videos. All contents are released under Creative Commons CC0, which makes them safe to use without asking for permission or giving credit to the artist"
openclipart: Drawings and clipart that are all in the public domain.
Pixnio: High quality public domain images that can be used freely for any personal or commercial use.