Foreign Law Guide: Iran. Marci Hoffman, General Ed., University of California, Berkeley Law Library. Describes the general sources of law and cites to specific laws on various subjects such as taxation. When you get the main Foreign Law Guide page, search for Iran.
Guide to Law Online: Iran, Law Library of Congress.
UPDATE: A Guide to the Legal System of the Islamic Republic of Iran (2011), GlobaLex by Omar Sial & Ershadul Karim.
UPDATE: An Overview of Iranian Legal System (2015), Globalex, by Maliheh Zare.