Ayatollah Montazeri is a senior cleric of Iran who was first chosen by Khomeini to be his successor. But his liberal views angered Khomeini and he was ousted. In recent years Ayatollah Khamanei, Khomeini's successor has placed Ayatollah Montazeri under house arrest. Mr. Montazeri is still the main opposition figure for Islamic reformists who believe in Islam but not in the Velayate Faghih. He is the most respected religious figure in Iran.

The following are some of the links about Ayatollah Montazeri.

This is the translation of the letter of Khomeini to Montazeri dismissing him as his heir. 
http://www.eurasianews.com/iran/montletr.htm
http://news.bbc.co.uk/hi/english/world/middle_east/newsid_273000/273884.stm
http://news1.thdo.bbc.co.uk/hi/english/world/s/w_asia/newsid_98000/98077.stm