The citation rankings from Science Citation Index are practically a requirement for university tenure these days. But the datqabase is so expensive that most libraries have to search on a for-fee basis. I did one recently that was $30 for 6 citations to one article.

Google Scholar service matches Thomson ISI citation index

The free Google Scholar service does as good a job as Thomson ISI’s science citation index for performing citation counts and could be used as a cheap substitute to the costly Thomson service, says a University of British Columbia professor. Thomson’s citation databases are accessible through the company’s Web of Science portal only by subscription, which can cost a university tens of thousands of dollars a year.

… Many of the journals indexed by Thomson ISI are not available electronically, or are available only by subscription, and so would not necessarily be picked up by Google Scholar. Dr. Pauly suspects that Google Scholar compensates for this by finding citations in the so-called “grey literature,” such as reports, books, conference proceedings and other items found on the web. The result is you can use Google essentially for the same purpose as you use Thomson ISI.