Collaboration tools or plugins?

My question is this: Are there any WP collaborative tools out there? The main feature I am looking for is a way to message other users from within the admin panel. Something that allowed for me to write a note, and I could either choose to have it show up for another specific user when they login, or perhaps a group note, where I can post a note that will be seen by any of the other users when they login. And replies would be possible between the users. A very basic bulletin board, even text only would be fine, in the back end to allow users to communicate, nothing full featured or bloated. A clean and easy message system in the admin panel only. Does anyone know of a plugin that would make this possible?

Thanks in advance.