There was also Meebo, which allowed you to login to all of them via a web interface (which I believe none of the messengers had natively) without installing the respective clients!

AOL had 'AOL Quick Buddy', a Java applet client and later AIM Express that used Adobe Flash.

I definitely used quick buddy from computers at my junior college (98-2000), but I don't remember using AIM express.

Ah, I never used AIM directly. Maybe that would have worked with ICQ too, though?

In any case, Meebo worked using the (back then) advanced, exciting magic of AJAX :)

I think it did at some point, but I think it was quite a few years after ICQ was purchased before the infrastructure merged.

I used the icq web interface for a few years. It was 'ok' it was missing a lot of features but worked in a pinch.

Meebo was sick. I remember they never made any money though as an IM webapp, and eventually fully pivoted to some kind of on-page ad toolbar that site owners would add to their sites...for some reason.

Adium! It really was a gem of an app. Much better than any other I’d used at the time (or that I have used since, actually).

Miranda was better.

Indeed. It was so nice to have a native app for work chat, since we could add Google Talk (XMPP) to Adium.

A pox on Slack.