Radio is a plugin for Bukkit which allows for player-created radios for the purpose of long-distance message transmissions. It is meant to be run on a server with a global chat limit (i.e. players can only talk to those nearby them), but this is not required for the plugin.
You can find stable downloads with detailed changelog information in the files section of the plugin's BukkitDev page.
Simply drop the latest .jar into the /plugins directory of your server.
Further plugin information can be found at the plugin's BukkitDev page.