Feathering the light that comes from your softbox is a very important tool/technique and as photographers we need to know how to do it.
So here are a few articles that will show you how:
From Strobist -
Feathering a softboxA PDF -
Feathering is ticklish businessAmherst Media -
FeatheringDigital Artform -
Feathered light from a grid (
be warned this article has some mannequin nudity)
;)