There are different metrics that can be used to rank journals. One widely used is the Impact Factor that measures the frequency with which the average article in a journal has been cited in a particular year.
For most metrics, an individual measure doesn't mean much, but can be used in comparison.
Want to expand your list of possibilities? Ulrich's is a great place to look or there may be directories for journals in specific fields.
Find out who has cited a known author or work (article, book, conference proceeding, patent, etc).
The h-index is an index that attempts to measure both productivity and impact. The index is based on a set of most cited papers (from an author or it can be calculated for a journal) and the number of citations that those publications have received in other people's publications.