Even before the World Wide Web emerged in the mid-1990s, scientists dreamed of having the whole body of scientific peer reviewed literature freely available without any hindrance.
In the "Open Access" scenario each published article can be accessed by any reader worldwide, a model which is in contrast to the established subscription system, where access is only provided to those people who are able to pay for an annual subscription, usually libraries.
The downside is that scientists have to use grant money to 'pay to publish' in the newer 'Open Access' model, and the up-front costs for publishing are quite high, often $1,000 and up, though it removes any barriers to reading what many believe should be publicly available material and in addition often provides for full use and re-use of the published output, facilitating developments such as data mining of knowledge discovery.
Slowly but steadily Open Access journal articles have increased, thanks to pressure from research funders such as the NIH and Republican resistance to House Judiciary Chairman John Conyers Jr., Democrat from Michigan, and his efforts to prop up legacy publishing. Specialized OA publishers such as the Public Library of Science and BioMedCentral (where you can even create your own!) have entered the market and more traditional publishers have also begun to offer OA alternatives when, as with OA publishers, the scientists essentially buy the copyright from the publisher.
Obviously not everything OA is peer-reviewed, given 25,000 journals now in existence, and an article published today in PLoS ONE shows a very rapid growth of OA publishing during the period of 1993-2009 due to those lower standards. Since the year 2000, the average annual growth rate has been 18% for the number of OA journals and 30% for the number of OA articles. This can be contrasted to the reported 3.5% yearly increase in the total number of peer reviewed journals. It was estimated that in 2009, articles in OA journals were 7.7 % of all peer reviewed journal articles.