Of all the tools I use, these are by far the least expensive and most useful. I use them for reflection, diffusion, blocking light, and even tabletops (I wrap cloth around them and secure it to keep the wrinkles out).
I use the 20-inch x 30-inch size, and to make them stand on their own cut them in half, and then tape them back together (I use gaffers tape, but you could probably use duct tape, too). This way they will fold like books and stay propped open to add fill light, just like I did in this photo to the left.
• You can buy foam board at almost any craft store (or department store where they sell craft items).