LiteManager OSX client and iOS client.
The «Send Message» mode allows you to send a simple text message to the remote computer. The message will be displayed on the remote computer screen, which will attract the attention of the user who is working with it at the moment.
You can send a message to a group of computers to do this, select the list of multiple contacts and send them the message. The user will not be able to reply to this message, to communicate, use text chat mode.
In all user sessions — send message to all sessions on the remote PC.