ICQ

ICQ was one of the earliest widely adopted instant messengers, built around user numbers, status indicators, and a surprisingly deep feature set for a service from the weird young internet.

It helped define the grammar of online messaging: presence indicators, alerts, contact lists, and identities that sat somewhere between anonymous and recognizably you.

Later services got bigger, but ICQ helped establish a lot of the habits they refined and made ordinary.

Its memory lasts because it feels like pure early internet communication: direct, quirky, and permanently accompanied by that sound.