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