"Open access (OA) is free, immediate, permanent, full-text, online access, for any user, web-wide, to digital scientific and scholarly material, primarily research articles published in peer-reviewed journals. OA means that any individual user, anywhere, who has access to the Internet, may link, read, download, store, print-off, use, and data-mine the digital content of that article. An OA article usually has limited copyright and licensing restrictions." (Wikipedia, May 15, 2008.)
Right now, there are two types of OA collections that primarily concern academic researchers.
Use the Open Access Webliography for comprehensive information on all things OA.