The pop-up flash found on many of the DSLRs is not that great. The light source is small and can be harsh to the subject, especially people, making them very stark and flat. Search on the web and you'll see many tutorials on ways to fix this, as well as links to retail solutions that are inexpensive and look professionally built. Now no diffuser available will make these pop-up flashes great, but it will improve their performance a bit.
My version on the DIY diffuser is based on the film canister idea but instead I used a travel toothbrush container I picked up at Walmart for 97 cents. I wanted to make mine where it wouldn't stand out as something other than what I meant it to be. It slides on easily from the side, is firmly seated in the front by a little groove under the flash and remains stable during use. In the picture you can see how it attaches as well as the with and with-out results. With the diffuser, you see the wall paint the correct color. It also works well with portraits.