Since
ICQ is made by
Mirabilis, run by Mirabilis, and is a
closed protocol, only Mirabilis can make ICQ clients right? Ah, but some
friendly blokes got out their
port scanners and
packet sniffers, and
documented the
ICQ protocol. Using these documents, more friendly blokes got their
coding hats on and started writing their own ICQ clients for operating systems that don't have
official clients from Mirabilis, or where the official client
sucks rock. One of the first was
mICQ, which morphed into
icqlib, and this is what many Unix ICQ clients are based on.
Here's a totally non comprehensive list of clients for various OSes:
AFAIK, no one has yet cloned the ICQ Server, so Mirabilis could break all these clients any time it wanted, by updating the server apropriately, and releasing a new version of the official ICQ.
Addendum: Guess what. In October 2001, Mirabilis / AOL gradually started phasing out the old ICQ protocol used by most of the clones. The new protocol, ICQ protocol version 7 is based on the AIM OSCAR protocol. Old ICQ clients (including Mirabilis 98a) will almost certainly not work any more. But not to fear, there's plenty of new clients around that work with version 7. Don't get too relaxed though. It'll happen again.