Coccinella is a free and open-source cross-platform communication tool with a built-in whiteboard for improved collaboration with other people.141 weekly downloads
Monster Communicator - an innovatory java communicator with lots of nice, never seen in other communicators, features like virtual blackboard. Supports Gadu-gadu, Tlen, Jabber. ICQ, MSN, AIM, Yahoo and Skype will come in future too. Go see screenshots.