If you want some carefully curated list for you discipline check out the below resources:
There are many places to find openly licensed multimedia to use in your courses. You can also use free library resources in your classes. And though library resources are in general copyrighted, you can use them in a zero textbook cost class because they are free for students.
You should evaluate OER as you would any other resource. Here are some questions to ask when evaluating a resource:
Examples of rubrics for evaluating OER:
Adapted from Iowa State University, University Library's Evaluate OER page.