ICQ is a
proprietary implementation of
Instant Message (
IM) technology. There is free implementation called
Jabber - it uses open protocol based on XML and free source clients.
ICQ is
closed source and it's
protocol closed also. Some people have dug it out using simple
network tools (such as
tcpdump).
Now (feb.2002) the most recent protocol version is v8.
Older versions are not supported by
Mirabilis any more. This means that older clients and free implementations of older versions (like
mICQ) could work only partly or have big delays (up to days) in deivering messages.
Mirabilis fights all other icq-cmpatible implementations of clients and servers with gateways.
Still, you could get a free ICQ server - IServerd and free gate to Jabber - icqtransport.
ICQ is not a secure protocol. There was a client (I do not know it's name, it was written by someone from Ukraine) which have had ability to send messages from any UIN number you like. Also you could intercept messages for other persons and get passwords with it. Instead of securing icq's protocol, Mirabilis insist on removing the WWW page with this software and calls it illegal.
There is Perl implementation of ICQ client for Unix console - vICQ.