selecting Box instead of Flat as projection, and then setting the blend value.
Blend 0.0 will not do any blending between box sides, higher values will give
a smooth transition between the sides
This is useful for mapping image texture without too obvious patterns in a
way that looks seamless, without the need for UV mapping. This works basically
the same as the mango node setup that was posted, just a bit faster:
http://mango.blender.org/production/blended_box/