IMSpector is an Instant Messenger transparent proxy with logging capabilities. Currently it supports MSN, AIM and ICQ to different degrees. MSN is the principle protocol, as it is these days the most popular. The only supported platform is at present Linux, but porting to different UNIXs should be trivial (the only platform-specific code is the routine which determines the original destination address).

Might be useful if you have/want to allow IM but need to keep tabs on it for anti-bullying/child protection purposes.