The Canadian Association of Research Libraries (CARL) has published a useful guide to altmetrics (pdf). Altmetrics are alternative metrics to measure the impact of research.
Also of interest, the Summer 2013 (vol. 25 issue 2) issue of Information Standards Quarterly from NISO covers altmetrics (direct link to the PDF of the full issue).
In the same vein, one can determine the impact of an institutional repository by visiting this page ranking repositories. The link send you to the Canadian listing, where my home institution’s Soectrum ranks fifth. In fact, I just learnt that I’m still in the top 10 researchers being downloaded from my University’s Institutional repository!