The following collections are "digital repositories." They are organized collections of "green open access" articles, which means they are articles which have been granted full license for public use, and are freely available.
Open Educational Resources (OER) are educational tools which are licensed for public use and are freely available to all users. These encompass areas such as Open Textbooks, Open Access Journals, and many other types of educational media which are legally available for free.
These exist as a response to the increasing cost of educational materials, such as textbooks which are often beyond the budgets of many students. Many educated professionals and academics have gathered together to make use of their own expertise to create these resources, so as to make sure that educational materials are available, regardless of an individual's ability to pay for them.
For more information, and a deeper explanation, read the Open Educational Resources Infokit.
This is a collection of textbooks that have gone through a peer-review process, and are considered to uphold a high standard of quality, without the expense associated with many commerical volumes.
Here is a brief selection of many useful and free online learning tools that can either be used independently or integrated into existing classrooms.