The European Commission recently released a report that finds that half of all articles published in 2011 are now freely available to read. The rise of open access journals, delayed free access in traditional journals (typically a 12 month embargo), and author-archived copies are all believed to contribute to the increased availability of articles.
Nature news article about this report: http://www.nature.com/news/half-of-2011-papers-now-free-to-read-1.13577
Link to PDF of report: http://www.science-metrix.com/pdf/SM_EC_OA_Availability_2004-2011.pdf
