The "stack all" button.
I'd been living in Providence for months, and was meticulously stacking each type of loot by dragging one item on top of the next.
And over again.
After stacking that 122nd Large Energy Transfer I, this can get pretty mind-numbing (not to mention painful on the hands, doing a repetative motion like that). So I got fed up with it, and went on to bitch on alliance vent about how they should really add an instant stack button. Once everyone had stopped laughing, someone pointed out the "stack all" button.