The liberal Andrew Sullivan?

Perhaps only slightly to the left of A.S., a contrarian leftist (most noted for favoring welfare reform and arguing for a new approach to equality on the left) - Mickey Kaus.

Further to the left, a serious independent reporter who has been touted as a future left pundit who nevertheless is potentially a war supporter - Joshua Micah Marshall.

Still further to the left, in The Nation territory (but scornful of Ralph Nader's presidential run, though not his politics), complete with occasional music writing (he's written a book about Bruce) that's often embarrassing - official pundit Eric Alterman.

Also, the weblog (or 'blog', you know that's what these things are called, right?) of a "progressive"-type left magazine, The American Prospect - Tapped.