Pros: Large collection of textbooks with published peer reviews by professors, does include some upper level/graduate textbooks, easy to use interface.
Cons: As the name suggests, the content is limited to textbooks.
Pros: Simultaneously searches multiple repositories, could give you good ideas of other repositories to look in.
Cons: Search interface is difficult to use, search results sometimes take you to the main page of different repository and then you have to redo your search, not easy to determine licenses for items.
The University of Oklahoma Libraries have compiled a comprehensive guide to many existing discipline-specific resources. You can review it here.
If you're interested in finding open materials that might be relevant to a text you're teaching, try using MERLOT II's ISBN search function. Just input the ISBN of the text you're currently using to see what materials could be supplemental.