How to make a message appear on the screen?
Yes, this would be fairly trivial with ssh to gain access to the Mac and then run whatever command you prefer.


Is there a way to add items to notification center through terminal or other means? - which covers SeNotifier and the more general purpose terminal notifier which both leverage OS X's notification center app for the GUI.
How can I trigger a Notification Center notification from an AppleScript or shell script? - which covers both AppleScript and Terminal scripts - either of which will work when called from ssh from Linux, Mac or Windows remotely. It also adds in Growl2 as a GUI for messaging on OS X.