http://www.imperium.com/relevantid.html

IMPERIUM:

RelevantID’s digital fingerprinting technology is used by survey researchers as an effective quality control that maintains the integrity of Web-based research. By creating a digital fingerprint of each computer, it identifies duplicate respondents who take the same survey more than once from the same machine. RelevantID flags a computer each time it tries to take a survey, so it is able to detect if multiple e-mail accounts are being used to take surveys from a single computer.