Who's Online free WordPress plugin

The “Who’s Online” sidebar widget shows a list of recently-active blog users. It has been built with Automattic’s P2 theme in mind, but does not depend on P2 functionality. The most recently active users bubble up to the top. After one week, users fall off the bottom of the list and are hidden until they return. As of this writing the list does not support live sorting; the user order is fixed, and users missing from the list due to inactivity will be added to the top if they come online.