America On-Line had that snazzy little sound. If I could find a freely available sound, I'd put a little flash button here that you could click and a link to an mp3. Baaaa-Riiing! And you had a message. It wasn't as weird as chat rooms and you could "IM" your friends.
Growing up, I didn't have America On-Line so I used Mirabilis ICQ (which was bought out by AOL at some point) and it's notification sound was much more annoying. It was this very high-pitched "Uh-Oh!" that is scraped into my mind as one of the sounds that should only be played for historical purposes. I will not post this sound a flash button, even though it is freely available. I really don't ever want to walk in someone reading my blog and having to listen to that sound. It was really that bad.
But that sound meant that your friend just sent you a message. And it was grand. ICQ also had this "real time" chat where you could directly connect with multiple friends and watch letter-by-letter as each other typed. It was phenomenal! This feature was also something I had only played with called "talk" which I had seen running Slackware Linux that allowed you to do the same thing in those classy large white terminal font letters on that night black screen. I remember trying to get a bunch of friends to all connect to my computer with similar black boxes so we could chat like that years later, but ICQ had become a giant by then.
And then for some reason, America On-Line opened up their instant messaging service so any internet user could connect to their instant messaging networks and have that same "Baaaa-Riiing" with their friends. I immediately installed their program and instant messaged a friend of mine and he was excited that I had become a "member" of his network. And this is how we do some of our communication to this day, at least a decade a go.
All images retreived originally from wikipedia.