I just tried out Talk and write for skype, a whiteboarding tool that uses skype for communication. It’s useful, but doesn’t have too many features. Good for two people to interactively discuss and comment on a document or diagram. It is windows only, though.
Bitwise IM also features a whiteboard. It is also free for personal use and is multiplatform (Windows, Mac and Linux).
DimDim also looks interesting, it bills itself as “The worlds free webmeeting.” I’m yet to try it though.
