The figure shows that over 100 million documents* (about 60% open access) were open through 5,000 content providers.The growth rates (22% for documents, 27% for content providers) are particularly impressive given the high pre-existing content rate.
In the article entitled “Dramatic Growth of Open Access” additional resources are mentioned such as: OECD iLibrary, Directory of Open Access Books, arXiv, socArXiv, bioRxiv, RePEC, Internet Archive
* BASE: Bielefeld Academic Search Engine
**Currently in BASE: 103,458,284 Documents of 5,028 Content Sources
Image and Data available here