You may be familiar with citators such as Shepard's Citations, which allow you to search for resources that cite to individual cases or statues, but did you know you could perform the same function on many humanities and social science journal articles?
Web of Knowledge (and Web of Science, available through the Marriott Library) allows you locate academic journal articles with cite to a single journal article. In addition, Web of Knowledge will display the "impact factor" of the article: the impact a single article has had on a particular subject area based upon the number of times it's been cited.