Flash Diffusers are one of the most common tools in the photographers’ bag; obviously, there is a great variety of them, starting from the expensive models sold by the most famous producer, to cheaper, but equally good models, like the famous Sto-Fen.
Anyway, it is very simple to build your own inexpensive flash diffuser, and you can find several tutorials on the Net; let’s see some ideas:

• Digital Photography School shows how to build a flash diffuser with a simple Airline Barf Bag
• Torsten Balle Koefoed has published detailed instructions explaining how to create an Origami-Flash-Diffuser
• ‘List And Diagrams’ makes a creative use of a common slide box to create a diffuser for its macro photography