Journal impact factors
Earlier this year, Eugene Garfield published a JAMA article (2006 Jan 4;295(1):90-3) providing historical background and discussion of the significance of the journal impact factor (Garfield was a co-founder of Science Citation Index from which the impact factors are derived).
There has been considerable controversy about the role of journal impact factors in scientific publishing, including critiques of its utility/validity in the faculty tenure review process and possibly "shady" activities of some journal editors in prompting authors to increase the number of citations to their journal before accepting a given article for publication (i.e. attempting to "boost" the journal's impact factor by mandating citation of its articles).
A quick PubMed search yields a number of articles describing these and other issues, as well as how these factors are being used in some areas of biomedical research (such as exploring how impact factors change once a journal becomes available online).
Science journals artfully try to boost their rankings, Wall Street Journal, June 5, 2006