There are different tools that can measure the impact and prestige of scholarly journals.
There is no single definitive tool for measuring a journals impact and we recommend utilizing as many tools as possible to determine the best journal to publish in.
Eigenfactor.org is a project of Bergstrom Lab at the University of Washington. The goal of Eigenfactor is to rate a journal's importance to the scholarly community. Towards that goal, Eigenfactor gathers data from around the web, including Thomson Scientific's Journal Citation Reports, to compute two numbers: Eigenfactor score (EF) and article influence (AI).
Eigenfactor score (EF) is a measure of the "journal's total importance to the scientific community."
Aritlce influence (AI) is "a measure of the average influence of each of its articles over the first five years after publication."
For more information on the methodology and data sources behind the rankings, see Eigenfactor.org's FAQ.
Google Scholar Metrics is designed to help authors "gauge the visibility and influence of recent articles in scholarly publications." Towards that goal, Google Scholar Metrics assigns publications a variety of rankings based upon Google's h-index.
H-index is based on h-core, which "is a set of top cited h articles from" a given publication. "The h-median is a measure of the distribution of citations to the h-core articles." Search results display h-index and h-median scores for publications based on the last 5 complete calendar years.
For more information on the methodology and data sources behind the rankings, see Google Scholar Metrics Questions? page.