This is about the closest thing, but it doesn’t display this information to site visitors other than you as admin.
http://wordpress.org/extend/plugins/wp-useronline/
I suspect you are looking for something that behaves more like a forum would, showing the users online and their names.
You might be able to pull this off if your blog is restricted to members only and you used that plugin combined with this plugin AND some template tweaking, but I’m no PHP expert.
http://wordpress.org/extend/plugins/members-only/
This tutorial discusses a member profile type page that is created by tweaking the author.php file and some other code. But it may not even be close to what you want, just throwing it out there.
http://www.cagintranet.com/archive/wp-membership-directory/
