There is a lot of talk about Howard Dean for DNC chair. I think this is based on his being seen as being from the "democratic wing of the democratic party."
How about Robert Reich?
His progressive credentials go back quite a way, and he has been very consistent with them.
His views are based on logic, and not partisanship.
He is very charismatic, which is important for the money raising required of a DNC chair.
My guess is he wouldn't want the job, but when has that stopped anyone from suggesting a candidate?
For more on Reich and his views:
http://www.robertreich.org/reich/biography.asp
http://www.issues2000.org/Robert_Reich.htm
http://www.backbonecampaign.org/cabinet/nominee.cfm?ID=99