Steve Hitchcock of the Open Citation Project has updated his excellent resource, "The effect of open access and downloads ('hits') on citation impact: a bibliography of studies."
Here is Hitchcock's introduction:
Despite significant growth in the number of research papers available through open access, principally through author self-archiving in institutional archives, it is estimated that only c. 20% of the number of papers published annually are open access. It is up to the authors of papers to change this. Why might open access be of benefit to authors? One universally important factor for all authors is impact, typically measured by the number of times a paper is cited (some older studies have estimated monetary returns to authors from article publication via the role citations play in determining salaries). Recent studies have begun to show that open access increases impact. More studies and more substantial investigations are needed to confirm the effect, although a simple example demonstrates the effect.
This chronological bibliography is intended to describe progress in reporting these studies; it also lists the Web tools available to measure impact. It is a focused bibliography, on the relationship between impact and access. It does not attempt to cover citation impact, or other related topics such as open access, more generally, although some key papers in these areas are listed as jump-off points for wider study.
From Harvard University Library's Office for Scholarly Communication:
The following journal publishers have formally indicated cooperation with Harvard's open access policies and have agreed that Harvard faculty who publish in their journals may deposit those articles in Harvard's DASH repository under the open access policy without modification of their publication agreements, attachment of addenda, or waiver of Harvard's prior license. We are grateful to these publishers for their full support of access to Harvard faculty's writings.
We expect to add additional publishers and journals to this list in the near future. Publishers interested in being listed here should contact us for further information.
Publisher/Journal Confirmed as of
American Economic Association June 5, 2009
American Mathematical Society June 11, 2009
American Physical Society April 9, 2009
Berkeley Electronic Press June 10, 2009
BioMed Central June 5, 2009
Duke University Press June 29, 2009
Hindawi Publishing Corporation June 8, 2009
Public Library of Science June 8, 2009
Rockefeller University Press June 29, 2009
University of California Press June 29, 2009