The Public Library of Science offers not just amazing peer-reviewed journal articles on a wide and growing range of scientific topics, it also provides a succinct definition of Open Access, which is nicely expanded at the Open Access Week web site. Once you've seen what PLoS offers, branch out completely by exploring the Directory of Open Access Journals (DOAJ).
Notice the statistics at DOAJ: As of today 319,631 articles are included in the DOAJ service, from 4370 journals. Do a search on Google Scholar - chances are, recent articles with high citation counts (i.e., many other authors cite those papers) are articles that were either published in an open access journal or have since been posted in an open access archive. Any search with a biomedical theme is bound to lead to BioMedCentral, with an abundance of open access research articles. More open access highlights will be forthcoming all week. For a detailed, yet highly readable explanation of open access, you can't go wrong with Peter Suber's account. Enjoy!