Wetpaint, the wiki editing and hosting service, added private and group messaging this morning. The new service allows users to communicate one-on-one just like e-mail, and gives wiki administrators a new way to communicate to those moderating and contributing to their pages.
Sending a message in the service is pretty simple. If you're signed in, just click a user's name to pull up a "send message" pop-up. If you want to send out a group message, just start typing in names and the service will pull them up (like Gmail does).