Annika Backstrom

The “Who’s Online” sidebar widget shows a list of recently-active 
blog users. It has been built with Automattic’s <a href="http://p2theme.com/" rel="nofollow ugc">P2 
theme</a> 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.

Works as expected

Who's reading your P2 blog right now? Keep track.

Thorsten Ott

Simple widget to extend P2s (and other themes) functionality and display recent @replies for a logged in user in the sidebar. 
It can be also configured to show direct replies to comments or posts without the @username indication.

Mention-Me Widget

11-year old plugin and it still works just fine! Sweet!

It works!

Simple widget to extend P2s (and other themes) functionality and display recent @replies for a logged in user in the sidebar.

Who's Online vs Mention-Me Widget