Licq is a multi-threaded multi-protocol chat client written mostly in C++.
ICQ internet chat client
eyeCQ is an ICQ client integrated in eyeOS as an iframe.
mICQ is a portable, small, yet powerful console-based ICQ client.
GnomeICU is one of the most popular UN*X-based ICQ programs in the world.
kicq is an ICQ (I seek you) client, specifically designed for the KDE desktop.
Net::Oscar project is a pure Perl implementation of the OSCAR protocol used by ICQ and AIM instant message clients.
YSM is an ICQ console client.