Phillip Smyth, who writes the blog Hizballah Cavalcade about Shiite militias in Syria, Iraq and elsewhere, has an extensive policy paper (more a monograph) on Shiite militias in Syria out via the Washington Institute. I haven't read it yet, but after browsing through the notes my expectations were raised.

The Shiite Jihad in Syria and Its Regional Effects
by Phillip Smyth, The Washington Institute for Near East Policy

Btw there is also an interesting posting from last month on Hizballah Cavalcade about a new Shabak militia in Iraq.