I thought people might be interested in a flash diffuser I made recently that seems to give nice light quality. I had two problems with all the previous flash diffusers I made: (1) inadequate light below the subject at 4:1 magnification or greater, and (2) hot or cold spots in the output that reduced the diffusion. Some people like John Hallmen have mastered the art of the beauty dish diffuser that does not have a cold (i.e., dark) spot in the middle, but I couldn't manage it.
My solution was to build a diffuser that fired the flash perpendicular to the plane of the lens and allowed it to bounce of the internal parts of the diffuser before leaving the front. Is it ugly? Yes, very ugly. But, it gets the job done.
The inside of this $1 plastic container is coated with aluminum tape to make it extremely reflective. The flash thus bounces off the far side of the container and reflects out through the sheet of white paper from all interior points at once. This design eliminates all hot spots and produces essentially uniform brightness from all points. The wrap-around shape also allows you to shoot at 5:1 without the problem of having the underside of the subject be too dark.
To make the channel at the bottom, I cut out a section of the original container and then made a new curved piece to fill in the gap. You want this diffuser to be completely sealed, because using bounce flash eats up the batteries fast, and any escaping light makes that problem worse. I took a piece of stiff manila folder and reinforced it with aluminum tape on both sides to make it stiffer and reflective.
This diffuser still doesn't do as well as the MT-24EX with a concave diffuser design at lighting the subject, but it's much cheaper when you already have a 580EXII or 430EXII in your kit. Also, you get single catch lights instead of dual catch lights, which some people prefer (I don't care personally, but I do care about spending the cash on the MT-24EX 
). I do not show my flash bracket in these pics, but it's of no consequence; any bracket will work if it can get your flash out to one side pointing across the lens rather than down the barrel.
My Current Flash Diffuser
by gatorlink, on FlickrAnother angle:
p2
by gatorlink, on FlickrA shot at 5:1 to show the adequate light below the subject and the uniform catch lights in the eyes with no hot spots:
