There are a number of search portals and repositories that compile free and openly licensed resources.
Type - Some repositories are purely Open Textbook Collections, whilst others cover varying types of open material such Full Courses and other supplementary material such as syllabi, case studies, images and videos.
Quality - Quality of material varies, depending on each repositories' requirements. Generally, OER affiliated with particular institutions with textbooks or courses authoured by faculty are of good quality. Other collections also stipulate certain requirements or peer review processes, ensuring a certain standard. However, care must still be taken to evaluate material:
Conditions of Use - Although most material listed in these search portals are openly licensed (can be remixed and adapted), not all are, and some are only free to use as-is. Generally, under the different Creative Commons licenses, attribution/credit must be given to the original authour, and material reproduced cannot be further copyrighted.