Mirabilis' ICQ is an InstantMessenger system. It can be downloaded for free: http://www.icq.com/download/
Being an older public system than AIM, it seems to have more users, but this is highly debatable.
People who use ICQ:
For those concerned about security, licq (now version 1.0: http://www.licq.org/) supports SSL connections for messages. SSL-encrypted chat+file xfer is not there yet though.
ICQ is really poorly written. Another example of WorseIsBetter. Not only do the clients crash a lot, and the user interface sucks, but the security model is ridiculous. It's almost entirely client-based. That is, if you wish to be invisible to another person X, X's client will "hide" you from her. However, it's easy to circumvent this. ICQ also won the prestigious Worse Website award in 1998. Naturally, I have ICQ running right now. [Might explain why I have Netscape 4.6 and Windows 98 on my machine too.]
The Mirabilis clients are terrible, I'll agree. The security model is not impossible, and works for the majority of people. (I leave that up to SecurityPolicy? discussion.) Look at the clones if you want to see stable clients. My client (licq) never crashes. --TaralDragon
Yes, I was coaxed by friends to use ICQ (if I wanted to have any kind of social contact with them, as they seem to never be home and use /dev/null as their answering machine dump device), and I agree it is crap (the software and most of the idea). I use licq, and while it is somewhat reasonable, the "stateless discussion" and continuous interruptions that it introduced to me are really annoying. The chat mode and file transfer is useful, but it takes very little firewall to deter them from working. --PierrePhaneuf
Update: I've become so mad at ICQ that I have stopped using it altogether. --PierrePhaneuf
See also InstantMessenger for other alternatives