I developed a very small plugin which displays on a web page the number of users currently online.
Take a look here:
From the readme:
"The OnlineUsers Plugin exposes the number of currently online users through HTTP.
Just open http://yourserver:9090/plugins/onlineusers to get it."
List of online users
Since version 1.1.0 it's possible to get the list of online users through http://yourserver:9090/plugins/onlineusers/list.
Standard behaviour is that you have to login in order to access this list. You can change that through the system property plugin.onlineUsers.list.disableAuth **(set it to **true).
If you change this setting, you have to restart the server (or reload the plugin, but I didn't try that).
Jar is in the "bin" directory.
The plugin is released under GPLv2 and has been developed by me at the Amiado Group.