Merlot is one of the longest running OER sources, and includes a wide variety of OER types, including presentatios, syllabi, animations, case studies, and more. Many of the materials include Editor and User ratings.
OER Commons also hosts a variety of materials as well as helping you find them, and has a built-in evaluation and ratings system.
Open Textbook Library collects textbooks that have been reviewed by faculty from universities and colleges in the US.
In addition to titles specifically labeled "OER", there are hundreds of Open Access Book Publishers that may have monographs on relevant subjects. A curated list is available at our Scholarly Communications and Open Access LibGuide.
As well, many OERs live in institutional repositories, and can be found by using search engines such as CORE.