Reppit

reppit does only one thing - it's close to being the smallest possible way to effect a change of the reputation bar (by switching your watched faction) whenever you gain reputation with someone.

There's dozens of addons doing this, this one simply sets itself apart by doing nothing else, doing it in less than 1k, and not getting into your way in any way, shape, or form. In fact, this description is already longer than the code it describes.