Since I use only natural light when shooting on my tabletop studio, I consider reflectors as an indispensable piece of equipment.  I use them to throw light into the dark side of my compositions. Comparisons of their use can be quite dramatic.  Luckily, the cost of purchasing them is small compared to other photography gadgets.  A 12″ reflector cost $10.99. Most purchased reflectors are circular with fabric over a flexible metal frame that can be collapsed into a much smaller size and come with a zippered bag for storage.  Usually, they are gold on one side and silver on the reverse.  The gold is useful for portrait work since it adds warm color along with the light, but I have never used the gold side with my tabletop studio because it can introduce unwanted colors into the scene.  I use a small, 12-inch reflector for adding light to smaller subjects, and keeping in mind the angle of the source light, I point the reflector into the shadow side, holding it as close to the subject as possible without getting into the frame.  Larger subjects need a larger size, and a sheet of white foam core can be pressed into service as a reflector, too.

Here is a comparison of what a reflector will do on a cloudy day indoors with south-facing window light.

The small 12″ reflector that I used can be purchased at  B & H Photo, and are handy for working outdoors in cloudy weather, too.