MERLOT
MERLOT (Multimedia Educational Resource for Learning and Online Teaching) is a curated collection of online learning and support materials, along with content-creation tools. It is led by an international community of educators, learners, and researchers.
The catalogue points to resources hosted elsewhere rather than hosting them all directly, and licence terms vary by resource, so check the terms on each item before you reuse it.
Related sources
LibreTexts
One of the largest open textbook platforms, covering higher education with some K-12 material. Hosts interlinked textbooks and reference material across science, mathematics, humanities, and social sciences. Mostly NonCommercial, so commercial use is ruled out.
Mason OER Metafinder
A discovery tool that runs a real-time simultaneous search across 21 different sources of open educational materials, rather than searching a static database. Useful for finding content to ingest, not a dataset in itself.
MIT OpenCourseWare
Teaching materials from more than 2,500 MIT undergraduate and graduate courses, published freely online. Includes lecture notes, assignments, and exams. Released under a NonCommercial licence, so it is free to use for non-commercial purposes only.
OER Commons
A digital library of over 500,000 open educational resources gathered from institutions around the world. Aggregates full courses, modules, lesson plans, videos, simulations, and assessments across every discipline and education level.