This sounds fancy, but the concept is simple. Teachers helping other teachers with shared lesson plans, resources, teacher-created materials (even textbooks), under a Creative Commons or other "open" copyright license. That means that you are free to reuse, revise and share these resources with others.
Edutopia OER Links and Helps
This is a portal site, very well organized, with links by content area to lots of great open resources. Take the time to watch the video, a great introduction to open resources.
Oxford University Open Resource Journals
Open access to quality research journals from Oxford University. Browse by journal title alphabetically.