ICQ and all it's features and add-ons made IM'ing so much fun. I remember the feature where you could type and your keypresses (incl. backspace) were immediately transmitted to the other side. You could also talk to random strangers and not be scared of frontal male nudity. Of course my IRL friends (I live in the NL) only discovered IM with MSN Messenger.

Funny how ICQ was, what, 20 years early to the IM revolution?

Slack nowadays is a mixture of IRC and ICQ. It was wonderful to be able to talk for free (well, paying dialup minutes) with the 3 friends you might had online, plus that guy on the other side of the world who you met through an ICQ number on a website :_)

ICQ + the friendly/outgoing/carefree attitude of those times

Beseen chat as well, same era.

And Palace Chat, though it was horrible even back then the memories make me laugh: https://en.wikipedia.org/wiki/The_Palace_(computer_program)

I still remember my 8-digit ICQ# by heart.