How to message any user on your LAN?

I have admin access to all (read most) the machines on my LAN. However, I wanted all users to be able to coordinate with each other. How should I approach this problem?

One solution which I can think of, but which I know doesn't exist in the exact form, is this:
messageping 192.168.23.45 Hi!
This would have popped up a message box saying Hi on the given local address.

There are quite a few softwares which allow you to chat over the LAN. But in the cases that I have seen, the user has to log in at the startup. Specifically I want to solve the problem: User Alice on address 192.168.23.44 wants to use MATLAB on 192.168.23.45, which is currently being hogged by Bob. I want a way for Alice to ping Bob asking him when will he be done using the machine. The solution should not involve any action other than a simple login at startup.

In Windows operating systems, you have the net send command.

As the documentation linked above says:

The Messenger service must be running for messages to be received.

Related: How do I get the "net send" messenger service working on Windows 7?

I may be mistaken, but if you use the Bonjour protocol with Pidgin, people can chat with each other as long as they are on the same network, with no need to login. Phone apps like WiChat can work with it as well on a Wifi network that's part of the same network.

In my work we use this software (it's freeware) for the same needs that you describe: Stickies

http://www.zhornsoftware.co.uk/stickies/

It runs on Windows 98, 98SE, Me, 2000, XP, Vista, 7 and 8 and no login is required, just install it in each machine and add the Windows Firewall exception (to avoid problems). Then go to the Stickies options and check "Enable networking"; go to the "Friends" menu and add every PC by its name in the LAN, or by its IP. And it's ready to work.

It's fast and simple to use: by doubleclicking in the "stickies" icon in the notification area a new blank stickie is opened, write some text in it, do right click in its header and with the option "Send to..." you can send it to one or some (or broadcast to all) of your "friends" in the LAN.