Citation analysis can be used to determine the citation impact of authors, articles, and journals. Beyond basic citation counts, there are measures such as the h-index and the g-index which are used to quantify the impact of an individual author.
Another method of assessing citation impact is to employ usage data such as the number of downloads for an article. Researchers are now looking at whether download statistics could be used to predict future citations (SLU).
To rank journals within a discipline or a sub-discipline, or to determine the average citation count for a journal use Journal Citation Reports (description here from Thomson Reuters).
There is a growing movement calling for the measurement of scholarly impact drawn from Web 2.0 data. Given that researcher participation in Web 2.0 tools is not insignificant, and is likely to grow as the "born-digital" generation move toward and into tenured positions, social Web metrics may become increasingly influential (Priem and Hemminger, 2010).
This chronological bibliography focuses on the relationship between open access and impact.
From Open Access sources, scholars find and often cite high-quality work.
Whether a college or university mandates a scholar to put his or her work in a publicly accessible Website, or the scholar is free to self archive as little or as much as he or she wants, the open access work is often cited, according to this article from PLoS ONE, an open access science journal.
A Thomson Reuters sponsored community forum.