Why do you need a program for it?

Just go to a command line and type

net send <machine> "<message>"

Or, create a batch file to do whatever you want to do.

You can find both programs and code to do this. Just do some searching.

Also, anyone wishing to receive one of these messages must have the messenger service enabled. They will also have to let it through the firewall... which IMO they are an idiot if they do.