Gary Fong has become a de-facto standard for speed light diffusion. The accessories made by Gary include the famous Lightsphere and lots of other small flash improvements.
Zond2 came up with a small and cheap flash application that mimics the lightsphere effect. The materials: A soda bottle, some foam cardboard and a starbucks latte cap.
I did not test this at home, but the concept looks really nice.
Zond was kind enough to share the process:
Start off with a cylinder cut from a 2 liter soda bottle
Place some foam paper on the back of the bottle and use a lid of a Dairy Queen MooLatte (O.K. Any other fine creamed beverage will do). Then use a foam board to create the bottom of the sphere. Cut it out a bit tighter then the shape of your flash.
This is how the interior looks like
And this is how it looks like assembled
And on flash.