Answer: ICQ

Although now an ubiquitous communication tool, instant messaging was not always a part of the typical computer user’s communication arsenal. In the early days of the computer revolution, there were several independent and primitive instant messaging-like tools on various Unix systems and other closed communication networks–the Unix tool “talk” is an early example of these instant messaging-like tools.

It wasn’t until the mid-1990s, however, that an Internet-wide instant messaging tool emerged in the form of ICQ. ICQ users were assigned a UIN (Universal Identification Number) that served as a sort of virtual telephone number/calling card they could share with friends and associates. Many of the features we take for granted in modern instant messaging applications (synchronized contacts and chat logs, offline messaging, and user-to-user file transfers) were first incorporated into ICQ.