The GURPS roleplaying system can potentially be used to roleplay in any setting, but you'll probably have an easier time of it if someone else has done the work of deciding how to apply the GURPS rules to simulate the various facets of the setting. This is the domain of sourcebooks. Particularly popular worldbooks from the third edition are: * GURPS Alternate Earths * Discworld * GURPS Reign of Steel * GURPS Supers * Hellboy * Lensman * Transhuman Space * Wild Cards The current edition of GURPS has the following worldbooks: