_Grey_

Instant Messengers

Miranda
The most configurable IM I can think of. Lots and lots of plugins available, including many protocols. Good for purists also, though highly configurable in graphics (plugins). Unfortunately, windows only.
Trillian
The most comfortable IM I know of. Quite stable in almost all cases and stylish, too. Windows only.
Gaim
Cross-platform IM, used on Unix-like and Windows Systems. Good for chatting and privacy :)

seems to speak of himself in the 3rd person. He knows the technical reasons behind this though.