Article-Level Metrics (ALMs) from PLOS for measuring the impact of research - ALMs provide a suite of established metrics that measure the overall performance and reach of published research articles.
http://altmetrics.org - "Altmetrics is the creation and study of new metrics based on the Social Web for analyzing, and informing scholarship".
http://www.altmetric.com/ - tracks what people are saying about research papers online "makes article level metrics easy".
James Pringle points out in his article: "Do Open Access journals have impact?" that Open Access itself does not necessarily equate to more citations - nor does it equate to fewer. He argues that increasing the potential journal readership does not change the fundamental value and relevance of an article in a journal to the work of a particular researcher. If any chosen access model allows the journal to be read by all or most of its intended audience, that audience will judge its relevance. Journals and other forms of scientific publishing will have impact based on criteria other than simply Open Access.
He makes clear that Open Access journals can have similar impact to other journals, and prospective authors should not fear publishing in these journals merely because of their access model.
By depositing your work in a repository or even more than one repository, you increase the likelihood of it being found by potential users as institutional repositories are easily searchable through the most frequently used search engines. This is why OA articles attract higher levels of downloads than subscription only articles. Another interesting observation is that in Hybrid journals, the inclusion of selected open access articles appears to have increased the the use of articles in older volumes of these journals.