The Messenger service in Windows was devised for this purpose, and works with or without the presence of Active Directory. However, there's been growing concern about security problems with the Messenger service in open networks; at one point there was a rash of "Messenger spam" being sent across the Internet to any net-connected computer that had the service running and wasn't behind a firewall. As a result, many administrators are now being advised to turn the Messenger service off by default.
There are third-party messaging solutions that can fill the gap, however, such as WinPopup (http://www.winpopup.com). If you're loathe to re-enable the Messenger service, this is one way to go.
This was first published in May 2004