There are many listings of OER online, often grouped by discipline. Here you can find selected listings of OER resources, as well as links to more extensive listings elsewhere.
One of the advantages of OER is they allow individuals to use and build on previously accomplished work without needing to reinvent the wheel. Here are links to other guides and listings with numerous OER sources for diverse topics.
The OER Commons is an online referatory with links to a wide range of openly-licensed resources including textbooks, labs, assignments, modules, and syllabi. There is a powerful advanced search option that allows instructors and students to identify resources appropriate to their discipline, purpose, and audience education level.
From the "About" page:
The Josquin Research Project (JRP) changes what it means to engage with Renaissance music. Our open-access website not only hosts an ever-growing collection of complete scores, but for the first time makes the music fully searchable: in a few clicks you can identify every instance of a given melodic and/or rhythmic pattern.