This is a server for GeoChat applications, written for OpenStreetMap, but usable in other environments.
Just copy the geochat.php file anywhere accessible from the web and point the clients to it. It stores messages in a MySQL database: write user credentials and database name in the geochat.php and run it from a command line to initialize tables.
Add a task to crontab or other scheduling application to run the script for a command line at least once an hour. It will clean expired messages and user accounts.
The API is described in the OSM wiki.
This script was written by Ilya Zverev and licensed WTFPL.