Repositories are online, digital archives that enable free and legal "Green" open access to research products.
There are two main kinds of open access repositories:
Institutional repositories (IRs) are online multidisciplinary digital archives of an organization’s intellectual output. Most university libraries maintain IRs as a service to their institutions. IRs preserve items for the long term and make them discoverable and accessible beyond the institution. Contents of repositories can usually be discovered through a Google Scholar search.
Subject or disciplinary repositories are online digital repositories available for the use of particular research communities. These can be maintained by scholarly societies, university libraries, or some other organization affiliated with the discipline. arXiv (pronounced "archive") is the longest-running and perhaps most successful example of a subject repository. Repositories can also be multidisciplinary but not institutional (like Zenodo).
The University of Saskatchewan's institutional repository:
Directory of Open Access Repositories is an authoritative directory of academic open access repositories: institutional or subject/disciplinary. Each repository has been visited by OpenDOAR staff to verify the information that is recorded in the directory.
The Open Science Framework provides this open source infrastructure, and many groups use it to produce their own subject repositories. You will notice that many play on the name "arXiv" - the most well-known subject repository. Although the word "preprints" is in the name of this platform it is often not just preprints but any version the author has the rights to post. Check with each repository for its policies.
Originally developed for CERN, Zenodo is now freely available as a repository for research products from any discipline. You can create your own collections and all publicly available uploads receive a Digital Object Identifier (DOI) to make the work easily and uniquely citeable.
This is a commercial repository that allows posting of all kinds of research outputs in an easily citable, sharable, and discoverable manner. All file formats can be published; unlimited public storage space and 1GB of private storage space for free. Some institutions and organizations outsource their repository infrastructure to figshare.
Confederation of Open Access Repositories is an international association that brings together individual repositories and repository networks in order to build capacity, align policies and practices, and act as a global voice for the repository community.