The "who's online" block in core does a nice job of this:

There are currently 135 users and 30 guests online.

It also gives a small list of the online users.
Jamie Holly
http://www.intoxination.net 
http://www.hollyit.net

On 5/4/2010 5:40 PM, Domenic Santangelo wrote: 
Other than the admin_menu module, do you guy recommend any method for showing (say, in a block) the number of anon/logged in users on a site?

Thx,
-D