Send short message to another Mac on the LAN
Messages app on Lion or Mountain Lion supports Bonjour discovery which lets computers discover other devices using Messages app on your local network without needing a central server setup.

Bonjour is not on by default, but you can find the option under Preferences, or simply ⌘+, to open up the panel. Select the account page, and you will find Bonjour in the sidebar.

Turn it on and you're good to go.

Now to retrieve the list of people on your local network, simply ⌘+1 to bring up the buddy list, it is also available under the Windows menu item.