There are a wide variety of finding tools available to locate both open access and open educational resources.
This is not a complete list, but should provide the tools needed to start searching for OER resources.
Keep in mind that some resources are available through multiple web sites and some are not.
DOAB Directory of Open Access Journals - Database of over 16,000 open access journals
Sherpa/Romeo - Contains information to open access publishers and journals from around the world
Open Access and Preprint Databases:
arXiv - Curated community supported research - sharing platform. Accepts a wide variety of research preprints primarily in math & sciences
bioXiv - Free online archive of biology and other life science preprints
Figshare - Repository for research output
medRxiv - Preprint server for the health sciences
PLOS Preprints - PLOS is a nonprofit, Open Access public server for scientific manuscripts
Research Square - Multidisciplinary preprint platform
Wikipedia - List of preprint repositories
Open Access Books:
Directory of Open Access Books (DOAB) - Open access peer reviews academic books on diverse topics
IntechOpen - Authoring platform and directory of science based textbooks and monographs
Open Textbook Library - Freely available textbooks and monographs
Open Access Theses and Dissertations - Global database
Oxford Text Archive - Repository of full-text literary and linguistic resources
Pressbooks Directory - Authoring platform and directory of open access textbooks and monographs
COAR Confederation of Open Access Repositories
OpenDOAR Directory of Open Access Repositories
ROAR Registry of Open Access Repositories
Bureau of Labor Statistics - Economic and employment data
Data.gov - "data, tools, and resources to conduct research, develop web and mobile applications, design data visualizations"
data europa eu - "The Official Portal of European Data"
Google Dataset Search - "discover datasets hosted in thousands of repositories across the Web"
IEEEDataPort - Datasets - "...offers free access to the datasets on IEEE DataPort"
UCI Machine Reading Repository (Beta) - 550+ datasets for the machine reading learning community
VisualData Discovery - find and share computer vision datasets
World Bank Open Data - Free and open access to global development data
BCcampus Open textbook Collection - Open reviewed textbooks on a variety of subjects and disciplines.
Directory of Open Access Books - 50,000 academic peer-reviewed
OAOpen - Based in the Netherlands "promotes and supports the transition to open access for academic books"
Open Textbook Library - "a comprehensive referatory that points to open textbooks by a variety of authors and publishers."
OpenStax - "publishes high-quality, peer-reviewed, openly licensed college textbooks that are absolutely free online and low cost in print."
Pressbooks Directory - directory of thousands of open access books
Resources by subject are available at the University of Iowa Library - Disciplinary Open Educational Resources
OER Search Engines
Mason OER Metafinder (MOM) - meta search engine for OER content
Merlot - "curated online learning and support materials and content creation tools"
OASIS - OASIS is a search tool that aims to make the discovery of open content easier.
OER Commons - " public digital library of open educational resources"
Skillscommon - an open library of technical, vocational, and professional resources
Blush - illustrations for everyone
Digital Collection - Library of Congress
Image of Empowerment - "...images celebrating women's lives and their work in 11 countries..."
Librestock - free photos from the top stock sites
Moving Image Archive - Free movies, films, and videos
nappy - Beautiful photos of black and brown people
Open Peeps - A hand-drawn illustration library
Open Verse - Openly licensed images
Open Video Project - a shared digital video collection
PICNOI - Free stock photos of a colorful world
Pixabay - 2+ million stock quality
Redefining women - the noun project
Unsplash - freely usable images
Freeplay Music - A large global music production website, access varies by use (educational/personal)
DRAM - "...provides educational communities with on-demand streaming access to CD-quality audio..."
Global Music Archive - A Global Music Archive