There are many sites that collect all kinds of OER, including short pieces like single readings, lesson plans, activities, quizzes and assessments, and more. You can search by discipline or a more specific topic:
Many states maintain repositories of OER materials. Maryland's is the Maryland Open Source Textbook Initiative, mentioned above, but other states have similar projects. Sometimes you'll see the same resources and materials across multiple repositories; other times, you'll find something unique. Here are a few of these statewide repositories from around the country:
There are also search engines and tools dedicated to finding OER, which can be faster than searching through multiple repositories or websites: