CU-SeeMe is a popular software for seeing, hearing and sharing over the Internet.  It can be used over the Internet or any TCP/IP network.  CU-SeeMe supports Macintosh, Windows 95/98, and Windows NT operating systems.  
....  Connections can be made with either IP addresses or simply e-mail addresses.  CU-SeeMe also supports color video and live audio.

CU-SeeMe Web Site