OER Commons
OER Commons is a digital library of more than 500,000 open educational resources gathered from institutions around the world. It brings together full courses, modules, lesson plans, videos, simulations, and assessments across every discipline and education level.
It was developed by the Institute for the Study of Knowledge Management in Education (ISKME). Resources carry a range of Creative Commons licences, so the reuse terms vary from item to item and you should check each one before you build on 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.
MERLOT
A curated collection of online learning and support materials, plus content-creation tools, led by an international community of educators, learners, and researchers. Points to resources hosted elsewhere across many disciplines.
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.