Using the number of sessions this block will display a short message showing how many users are currently active on your site. It has a built in style guide which can be disabled and the message it displays is configurable.

--- Changes 1.1 ----

The Registered user functionality has been disabled in the core code. The process used to determine a registered user relied upon some php file based functions and some service providers have these disabled.

I am working on re-implementing this feature in another manner.