This plugin counts current online users and overall users on Your page.
This is simple counter of users which are on Your page at this time (within time which You specify). It also counts overall how many users was on Your page all time. You can display those values anywhere You want on Your page. Now there is also simple sidebar widget which You can add to Your page's sidebar and it displays those counters.
If You have any problem with plugin after upgrade to newer version, please first turn plugin off and than turn it on again in wp-admin panel if you upgrade from version earlier than 1.5.3 to this one or newer than Your widget will probably dissapear from sidebar and You need to add it to sidebar again in same place