Can you send a message across the network? There used to be the ability, perhaps before OS X, to send messages accross the local network, which displayed a popup on the receivers machine with the message. Is this still possible? Or is there another way to send a message to another member of the network?

If this isn't possible, could a text file be sent, with a message, to a recipients Public folder?

Preferably with the terminal.

(I realise there are lots of aps which do messaging, but I was curious if this was possible without installing software or setting up iChat etc)
According to this metafilter post you can send a message using Applescript (pre-installed on every Mac) and Growl (old version is freely available).
The relevant part is quoted hereunder.

tell application "GrowlHelperApp" of machine "eppc://xxx.xxx.xxx.xxx"
register as application "myapp" all notifications {"message"} default notifications {"message"}
notify with name "message" title "title"   description "description" application name "myapp"
end tell     


Here xxx.xxx.xxx.xxx should be substituted by the IP address of the sender.