Huge searchable database of content you can use to instruct your students. Search by grade, type of resource, subject, and specific standard. Remember to check the box to search only free content.
Math, science and English content for teachers to use with students. Includes articles, activities, lesson plans, practice questions, and assessments. CK12 also has its own classroom platform or items can be assigned to Google Classroom.
Curriki has been part of the OER world for many years. They have built a platform for teachers to easily search their bank of teacher-created materials and also share their own with them Curriki community.