“A journal’s Eigenfactor score is our measure of the journal’s total importance to the scientific community. With all else equal, a journal’s Eigenfactor score doubles when it doubles in size. Thus a very large journal such as the Journal of Biological Chemistry which publishes more than 6,000 articles annually, will have extremely high Eigenfactor scores simply based upon its size. Eigenfactor scores are scaled so that the sum of the Eigenfactor scores of all journals listed in Thomson’s Journal Citation Reports (JCR) is 100.
…
A journal’s Article Influence score is a measure of the average influence of each of its articles over the first five years after publication. Article Influence measures the average influence, per article, of the papers in a journal. As such, it is comparable to Thomson Scientific’s widely-used Impact Factor. Article Influence scores are normalized so that the mean article in the entire Thomson Journal Citation Reports (JCR) database has an article influence of 1.00.”

For example, here are the ISI-indexed journals in the AI subject category ranked by the Article Influence score for 2006.

The site makes good use of GoogleDoc’s motion charts to visualize the changes of metrics for top journals in a subject area. You can also interactively explore maps that show the influence of different subject categories on one another as estimated from journal citations.

The details of the approach and algorithms are available in various papers by Bergstrom and his colleagues, such as

M. Rosvall and C. T. Bergstrom, Maps of random walks on complex networks reveal community structure, Proceedings of the National Academy of Sciences USA. 105:1118-1123. Also arXiv physics.soc-ph/0707.0609v3 [PDF]

(spotted on Steve Hsu’s blog)

Thomson Reuter’s journal impact factor is a measure of the frequency with which the average article in a journal has been cited in a particular year. The 2007 impact factor is computed as the citations received in 2007 to all articles published in 2006 and 2005, divided by the number of “source items” published in 2006 and 2005.