IP Messenger is a free and open source pop up style LAN Messenger for multi platforms based on TCP/IP (UDP). It doesn’t require server machine to operate. It is very useful product for offices, where are large number of computer connected through LAN.

IP Messenger allows you to send messages via the IP number of computers within designated blocks of addresses. If you own thousands of IPs, you can send messages to thousands of computers. This product is only designed for use within your IP scope allocated to your corporation. Real-time stats show you successes and failures.

Pros:

• Simple, lightweight, and compact size.
• Very easy installation and configuration.
• File and folder transfer.
• RSA/Blowfish Encryption of communication data.
• Multi platform (for Windows, Mac and Unix).
• Command line support (eg: ipmsg.exe [port] /MSG [/LOG][/SEAL] <hostname or IPaddr> <message>
  example: – C:\> ipmsg.exe /MSG /SEAL localhost Hello.

Cons:

• Little help.
• No smileys supported in messages.

This product is recommended for small and large business having own LAN and WAN.