Open-access repositories or archives are intended for researchers to contribute the results of their research. This builds up an information and knowledge bank that is easily, and in most instances, freely accessible to everybody.
Repositories or archives are either organized by subject area or certain institutions maintain archives which cut across different disciplines. For instance, there are 78 open-access repositories in Japan according to OpenDOAR, most of which are institutional ones, with only a few classified according to subject area. A typical example of an institutional archive is the Department of Energy (USA) information bridge, which provides free public access to over 200,000 full-text documents. One of the oldest repositories arranged according to discipline is arXiv, which started off as a physics archive but now extends to mathematics, computer science and other disciplines. It hosts over 600,000 e-prints.
Archives may contain raw or processed data in any format, preprints or post-prints, theses or dissertations, and in general any digital file, including software. No peer review is performed for the contents of the repository but authors have the option of contributing post-prints of peer-reviewed articles, provided they obtain the permission of the publisher. The copyright is generally retained by the author except in the case of post-prints (reprints), wherein the original publisher may already hold the copyright.
Open access archives are most useful when they comply with the Open Archives Initiative (OAI) protocol to harvest metadata. Such archives are interoperable leading to greatly increased ease of access. Every research institution or university should strive to build and maintain its OAI-compliant repository.
Listings of open-access repositories can be found at the Directory of Open Access Repositories: OpenDOAR and the Registry of Open Access Repositories: ROAR. Peter Suber maintains a list of the listings of OAI-compliant archives which can be found at http://www.earlham.edu/~peters/fos/lists.htm#archives
Open-access archives can be a rich source of information, and can be indispensable in situations where the material is not available elsewhere. The proliferation of open access archives would benefit both the research community and the public who usually fund the research indirectly.