I use many, many addons (oh, how I loathe major patch days!) but these are the essential ones:

Trinity
EngBags (the benefits of both separate bags and all-in-one bags)
Jamba (would hate to multi-box without it)
FuBar (and its addons)
Carbonite (extra useful when levelling for the Nth time!)
Postal (easy mailbox access)
Outfitter