Open Education Europa contains a very useful list of European repositories of OER classified by country, language and subject among other categories.
If you are looking for images, Photos for Class allows you to directly search properly attributed, Creative Commons, age-appropriate images from Flickr;
Unsplash contains photos licensed Creative Commons Zero, and Pixabay gives you access to hundreds of thousands of public domain images, including vectors and art illustrations.
What websites would you recommend to other course participants? Help us crowdsource a list of must-go online places: add at least one link to your preferred repositories/sites in the forum below. Ask your colleagues at work to give you a hand.