| 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. | 
