a couple other options
There is the Java ICQ Client - go icq! very handy when not at your own computer and still want to icq someone.
http://go.icq.com/
Jabber open source and secure
http://www.jabber.org/about/overview.html?...f7d13ecf2cfc93e
Instant-T
http://www.interactiveni.com/IMSite/imfreeware.htm
Omni, (no longer developed it appears, but might still be useful)
http://www.emphatech.com/index.html
iGo Incognito, a secure IM.
http://www.igo-incognito.com/overview.html
Quicksilver Instant Messenger
http://www.qsmessage.com/
Encrypted AIM
http://www.johnytech.com/home.asp
IMCI Secure IM
http://www.imici.com/
and for Alternative OS's, BSD, Linux users there are;
Kopete, is a flexible and extendable multiple protocol instant messaging system designed as a plugin-based system.
http://kopete.kde.org/
LICQ; Licq is a multi-threaded ICQ clone written mostly in C++. It uses an extensive plugin system to manage many different functions. The main gui is written using the Qt widget set. Licq is distributed under GPL with some special QPL exceptions for Qt.
http://licq.org/
GnomeICU;
http://gnomeicu.sourceforge.net/
EB-Lite
http://www.everybuddy.com/eb-lite/