One of the alternatives is Softros LAN Messenger.
Pros:
- It supports Terminal Services (great for large networks)
- All messages are encrypted by secure algorithm
- It sends messages to offline users
Cons:
- Application has 30-day fully functional trial
- Simple user interface
- Online/offline/busy status
- Net send command doesn't work in latest Windows systems. Softros works for XP, Vista, 7, 8, and 8.1
- Full version is about $12.95/12.48€ per copy