Impact Factors & Journal Rankings

Eigenfactor 

Eigenfactor analyzes journals and articles by looking at patterns networks of citations over time. This resource ranks journals and shows their influence in particular subject areas as well as showing visualizations of various data points. Data comes from Thomson Scientific's Journal Citation Reports.

Google Scholar

Google Scholar scans the citation information for scholarly articles and links out to other articles that cite a particular article.

Journal Citation Reports 

Journal Citation Reports provides easy access to data that helps you evaluate and compare scholarly journals and their impact factors. JCR uses citation data drawn from over 8,400 journals and 3,000 publishers worldwide and includes virtually all specialties in the areas of science, technology, and the social sciences.

Publish or Perish

Free software that allows a user to enter a citation, author, or journal name and obtains a variety of metrics using data from Google Scholar. Metrics used include the h-index, g-index, and e-index.

Scimago Journal & Country Rank 

This interactive site uses data from Scopus to provide journal citation information. Additional features include tools to judge research by nation, including rankings and concept maps to type of research.

Scopus 

Scopus identifies scientific articles in over 14,000 peer-reviewed journals from more than 4,000 international publishers. Selecting "References" in the search options will bring up a list of articles that cited the search terms. Also has options to see what other articles cited a particular articles and citation patterns over time.

Web of Science
Identifies articles in science, social science, and arts and humanities, and the articles that cite them. Database allows searches for items that have cited another item as well as visual mapping citations. Journal Citation Reports (under additional resources tab) gives journal rankings, impact factors, and other metrics.