Thomson Scientific (formerly ISI, Institute for Scientific Information) analyzes journal citation patterns to determine "Impact Factors" and "Half-Life" values for thousands of journals. These values are used to measure the importance of individual journals relative to other titles in their field. The annual Journal Citation Reports for Sciences and Social Sciences journals compile these data into ranked lists of journals.
UCSD now has electronic access to the Journal Citation Reports! (2007 - )
The UC San Diego Library also has previous years of the annual print/microfiche reports, which may be useful if you are looking for pre-2007 data.