LanTalk LAN messenger is designed as a full-featured, robust replacement for Winpopup messenger and Net send command. It's simple to use and requires virtually no setup nor administration. The program sits in the system tray until a LAN message arrives and can even be activated by a hotkey or double-click.
The unique feature is that you can grant message sending privilege to your computer only and the rest of the workstations will be able to receive the messages but won't be able to send them.
6.50 USD per user for up to 49 users, price goes down at higher quantities.