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.