Leader of Iraqi resistance faction the Mahdi (sometimes spelt Mehdi) army - named after a Shi'ite leader Ali Mehdi.
al-Sadr is a low-ranking Shi'ite cleric with a lot of support amongst Iraq's poor. He's been getting massive approval ratings in recent polls and is the second most popular person in Iraq, even among members of the rival Sunni religious denomination. This is probably because he is seen by many as the figurehead of resistance to the US occupation.
The US has tried to capture then to kill him, leading to clashes with his militia. He opposes US plans for the future of Iraq. Some commentators allege that he is allied to the Iranian government whereas others see him as a popular local leader.
Ironically, the US renamed Saddam City, a slum in Baghdad, as Sadr City after the occupation began - naming it after al-Sadr's father.
anti-war liberals and arab sympathizers may call him a resistance fighter, but he is a terrorist pure and simple
muqtada al-sadr is a shite (not shiite)cleric