Of all the DIY projects we write about, light modifiers are still my favorite. I have a nifty collection of flash diffusers and unusual lighting tricks that I think you'll want to keep in your back pocket.
The DIY Flash Diffuser with Paper and Rubber Band is one of the coolest and simplest devices to date. All you need is an old press release and a rubber band.
And don't forget about this Ring Flash Diffuser for a compact camera. Finally, a legitimate use for styrofoam.
Here's a collection of Five Poorman's Lighting Mods from DIYPhotography.net. I think the barf bag flash diffuser might be my favorite from this list.
And finally, one of the all time classics is to use a windshield reflector from your car as a fill light for outdoor portraits. My favorite of these reflectors (designed to keep your car cool on hot days) has white on the inside and silver on the out. This gives you two surfaces to choose from depending on how intense your fill light needs to be. Give it a try!