I took an old plastic film canister - a not quite opaque white one - cut out a notch large enough to fit it over the pop-up flash on my camera and ... voila. A simple DIY flash diffuser.
Here's a sample of the difference it can make. It's subtle, but significant.
This is without the diffuser.
And this is with the diffuser.
And for another of my DIY experiments - here is a filter made from a drain trap.