If you don't know what a flash diffuser is and you're wondering if you need one: YES! Absolutely. It makes the flash look much less harsh, and especially people will look much less like deer in headlights ^^
I usually avoid using flash at all but sometimes it's the only option.
Since I don't have an external flash yet and couldn't find any good diffusers for the built-in flash, I've tried out various means of diffusing it with rags, cigarette boxes, plastic bags, cardboard etc.
But recently a friend asked me to take pictures at a club, and it doesn't make a very professional impression to have a heap of rubbish tied to your camera when you ask strangers if you can take pictures of them :P
So I took a walk through the house to find something neutral and tidy looking that would work.
Find an empty shampoo bottle and hold it in front of a light source to see if it's REALLY WHITE (a very slight "warm" tint is ok when you're photographing people but a neutral white is best)
Also watch the labels - they need to be removable, not printed on.
Clean the bottle and get rid of the labels and the cap.
Cut a hole into the bottle top.
it needs to be big enough for the flash but tight enough so it doesn't keep falling off all the time.
Try if it fits a couple of times, and if it fits well, *slightly* melt the edges with a lighter so you won't scratch your camera with it. (Shampoo bottles are usually made of very soft plastic but at least it'll look better)
You're done! Now take a couple of pictures with and without the diffuser for comparison.
Sorry about the dirty mirror and the henna on my head xD
I wanted to spare you more pictures of myself, so here's some random shiny alcohol-related objects.
I'd normally use a lightbox for that but with the lightbox I don't use the flash of course so that wouldn't make sense ^^
Yes, the glass looks "worse" with the diffuser, but that's exactly the point! That's what will make peoples' eyeglasses and shiny foreheads look less reflective :P
And you can see that the shadows have softer edges.
If you like to experiment, you can also use colored bottles for colored flash effects.
Here's more random stuff photographed with the red diffuser
... and with a blue one.