From Cynetic Technologies:
Network Chat is a LAN chat utility that offers a clean interface, emoticons, private messages, sounds, and privacy all in an Internet-style chat window. Included with the package are several .WAV files for use as notification, but any .WAV file can be used. Network Chat can detect the names of all the computers on your network (within the same workgroup). You can also send messages to other users using their IP address, which also works over the Internet too. You can now send private messages as speech messages, which Network Chat will read out (Windows XP/Server 2003 only).