Light reflectors are an absolute must have tool for anyone looking to improve the quality of their photographs. With a good reflector, you can grab a chunk of light and point it exactly where you want it. If you want a colored light, use a colored reflector. The collapsible reflectors are super handy but I use several modifiers that are essentially free. Here are a few of my favorites:
Corrugated cake boards: We ate a cake recently and what do you know, it came with a free gold foil light modifier! Light bounced off of gold foil does wonderful things for skin tones. My 9" round reflector is perfect for isolating a little warm light on a subject's face to quickly make it the focal point of an image.
Old, low thread count white sheet: Draped over a bush, laying on the ground, hanging from a tree... This thing is a gigantic shadow eraser. It also does a fabulous job of softening any light projected through it too. Putting a flash on a stand behind a hanging sheet is a great way to mimic the light from a soft box.
Underbed gift wrap storage container lid: I have a big, flat, clear, Rubbermaid container that I keep my light stands, umbrellas, tripod, flashes and modifiers in. It has a white lid that I use all the time as a bounce surface for both flash and ambient light. The only thing that would make it handier is if it came with wheels and a handle - I'll fix that.
I could go on because I end up finding and using some kind of free reflector on just about every job I shoot. Most locations have good light reflectors lying around all over the place. Look for a sidewalks near shade (just be careful with that up-light, it's not always flattering), awnings, reflective windows, white walls, a sheet of paper, a box of aluminum foil, wall mirrors, compact mirrors, car mirrors, (drape your cheap sheet over the mirrors for a stronger but slightly diffused light), a tanless fat guy with his shirt off... Ok, skip that one, but you get the idea; Free reflectors are everywhere.
To use them, just decide what you want to be the focal point of your image, position your subject in neutral light, then take a reflector and bounce additional light on the spot you want your viewers to see first. There are a thousand other uses for reflectors, but that is the purpose I grab one for most often.
Best of all, you now have an excuse to indulge in that big chocolate cake with cream cheese frosting you've been craving. No, I'm not talking about turning yourself into a tanless fat guy; There's a gold cake board reflector under there, remember? Just be careful when you slice it so you don't tear up your golden ticket to free light. Enjoy!