IRC

Internet Relay Chat (IRC) is a form of real-time internet chat. It is designed for group communication in real-time discussion forums called channels. One could also chat with another user or transfer files using DCC.

History
Even though IRC chatting wasn't widely used by the public until the 1990's, it's been around since the early '60s and even predates the creation of the internet by decades.

Early chat development
Chatting was originally created in the '60s by connecting two electric typewriters to the same computer. Originally, it could only be used to send messages between two terminals in the same room. Eventually chatting grew to to be capable of sending messages within a building, and even between buildings that were close together. Chatting also evolved to include the ability to send messages to multiple users, but only if you specified each one.

EMISARI
In 1971, Murray Turoff created the Emergency Management Information Systems And Reference Index (EMISARI) for the use of the US Office of Emergency Preparedness. It featured the ability to hold over 30 users at the same time, could send messages to a central terminal using long-range telephone lines, and introduced the familiar notification when people entered or exited a chatroom.

PLANET
The Planning Network was developed in 1973 by Jacques Vallee, Roy Amara, Robert Johansen, and other developers from the Institute of the Future. It was meant for the use of government and industry planners, people who would most likely have little to no experience with computers. Because of this, PLANET was designed do be as easy to use as possible.

EIES
In 1974 Murray Turoff, the developer of EMISARI, expanded on his existing system in order to further explore the potential of computer conferencing, which would be called the Electronic Information Exchange System. Funded by the National Science Foundation, the finished system could accessed by a telephone line or computer network and was hosted by the Computerized Conferencing Center of the New Jersey Institute of Technology. At its peak, EIES had more than 2,000 subsribers, including IBM and NASA. EIES was used to aid in research, conduct conference sessions, and was even used by professors to deliver classes. It stayed in use for 24 years, well after the internet was popularized, until it was systematically shut down in 2000.

CompuServe CB
CompuServe CB was a program developed by Sandy Trevor "over a weekend on [his] kitchen table”. It primarily featured technical support channels for various companies that developed software, such as Microsoft or Adobe. It featured the ability to hold up to 300 chatters simultaniously, encrypted channels, and special enhancements to support the playing of multi-user games.

The birth of IRC
Modern-day IRC, the most widely used online chatting system in the world, was a developed by Jakko Oikarinen, a graduate student at the University of Oulu in Finland. After development had been completed, Oikarinen coordinated with his friends at other universities in Scandanavia to create a massive IRC network between university computers.