AOL Instant MessengerAOL really dropped the ball. This should really be what people still use today on their phones. Unfortunately, AOL has made some really bad decisions. Anyways...
AIM was what people used to talk with each other and leave each other messages. It was really popular with the college kids. There were other alternatives, like IRC or MSN Messenger or Yahoo Messenger, but AIM was top choice. It was text messaging before cell phones were common.
AIM and such programs eventually added more and more features. Everything from group chat, voice chat, video chat, file sharing, etc. Skype is probably the result of all these messenger services. In fact, MSN Messenger was faded out and replaced by Skype.
There were even programs that linked all these messaging programs together, like Trillian.
There were hacks that got rid of all the ads, and hacks that let you clone AIM so you could use multiple user names.
AIM was even influential in Siri. AIM had a bot where you could ask it several questions. You could ask about movie times, tickets, etc. It even had smarta$$ remarks if you asked smarta$$ questions, just like Siri.
Funny thing, I still have have AIM running, but no one is on. It still works (though maybe not supported). I am waiting for someone to log back on.