Heather Morrison's blog post the June 30, 2012 Dramatic Growth of Open Access http://poeticeconomics.blogspot.ca/2012/07/inevitability-of-open-access-june-30.html provides an excellent summary of open access search engines, world archives and the recent growth of their content. Here is a snap shot of the figures:

PubMed connects with over 3.5 million documents; arXiv, 750,000; Social Sciences Research Network 350,000; and RePEC, over a million. There are more than 2,000 open access archives around the world! DOAJ now has just under 8,000 titles, and is adding at a rate of 3 per day, and the newly launched Directory of Open Access Books already lists more than a thousand titles from 27 contributing publishers. The Electronic Journals Library lists over 35,000 journals of interest to academics that are freely available, and is adding 15 titles per day. The Internet Archive provides access to 670,000 movies, 100,000 concerts, 1.3 million audio recordings, and 3.5 million texts.

We also have our own treasure Trove where you can find and get over 302,682,563 Australian and online resources: books, images, historic newspapers, maps, music, archives and more.