It may not be of particular use for multiboxers, but they have proven invalueble nonetheless:

Mobmap
Ingame Database of NPCs, recipes, drops, ... Provides a nice map overlay as well as textual info on where to find whatever it is you're looking for.

Lightheaded
Provides all the comments gathered on WoWhead concerning quests. If you ever get lost, can't seem to find that one particular item, Lightheaded will get you going once again.