Introduced In: 10.2 "Jaguar" (2002)
Though far from the first instant messaging client, Apple's iChat was one of the most robust messaging softwares available. Introduced in '02, it was the first official AOL Instant Messenger client available for Mac OS X. Unlike other clients available for Mac, iChat was fully integrated with Apple's Address Book and Mail software, creating a trifecta of communication tools.
Before other chat-client companies introduced video chat, iChat supported it with iChat AV. It became super simple to have a video conferences with friends and family. Later versions extended support for other popular messaging services like Google Talk, Windows Live Messenger, and Yahoo! Messenger.