This post is not the one on Shi’ism I promised below, but it is still related to Shi’ism.
Roy Mottahedeh has a great piece on the nature of the Shi’ah groups in Iraq.
Juan Cole has a short summary of the relationship amongst Shi’ah groups in Lebanon, Iraq and Iran.
Both men are incredibly knowledgeable and extremely lucid writers. While both are predominantly political, they do give a surprisingly good idea of the religious landscape as well.