This plug-in allows you to create and edit cubemaps.
With "Extract cubemap region" you can extract a region of the cubemap, which can then be edited as a normal 2D image.
With "Merge cubemap region" you can merge the edited region back into the cubemap from which you extracted it.
The cubemap is a 4*size by 3*size image, consisting of 12 square tiles of size 1*size, of which 6 make up the cubemap. Those 6 tiles are laid out as follows:
The other 6 tiles remain untouched by the plug-in.