Pidgin was decent, but remember Miranda? The community around it was fantastic. The plugin system was an absolute blast. Not only there were plugins for any communication network you could think of, the UX was fully customizable.
At one point, somebody even bothered to implement the ICQ flash animations. There has not been anything like it ever since.
This takes me back. Y'all made me recall running Trillian Pro with a cracked loader. Oh, and what about DeadAIM?! I feel ashamed (as a current FOSS fan) that I didn't switch to Pidgin until after pirating all the popular AIM alternatives.