ICQ is a propriatary instant messaging program, first developed by the Israeli company Mirabilis. It uses AOL's OSCAR network protocol for sending the text messages. To identify peoples, it uses numbers, calles UIN(Unified Identification Number). But it is to connect an account with an e-mail address, so that you can login with you e-mail.

Features

Offline messages
multi user chat
file transfer

Features not yet supported by Kopete

games
sending SMS

Criticism

Privacy: according to the security analyst Jeffrey Carr, the russian secret service could gain access to chat protocols. For further information see here
Spam: there is a lot of spam (unwanted advertisement or viruses) in the ICQ network
Copyright: according to the ICQ Acceptable Use Policy, you allow ICQ to use posted material in any manner they want