BeeNut is the first interlopable IM client that I've really found tolerable, I've tried Trillian, Gaim, IM2, Meca, and a number of other clients. BeeNut has a pleasant GUI, it supports docking and autohide (something missing from every other program other than MSN with MSNShell), it also supports by default notification if a contact closes the conversation window. It doesn't (on the MSN side), have all the features that can be found in Mercury or aMSN, but the features that are supported are more than adequate. A couple of the features were a bit hard to figure out, but a bit of looking around will reveal them. The IRC component is a bit clunky but it does the trick.