In Mac OS you can use Boin

In Mac OS you can use BoinxTV. This program do exactly what you need. For Windows the only one I know is Adobe Visual Communicator. This program is not as powerful as BoinxTV, but with some extra effort you can manage a good show. To stream live with BoinxTV you need to download Camtwist and subscribe to any live streaming website (such as Ustream) I don’t know how to stream live with Visual Communicator and according to Adobe you need Flash Media Server (No idea of how this works)