A Plain flat white paper plate. Yes, a paper plate. If you're out and about looking to take photos of insects, flowers, or even some small reptiles a white paper plate helps for an isolated white background. You can also use it to redirect some light on your subject. For the paper plate make sure it has a matte finish to prevent unwanted glare. In this photo I used the paper plate as a background and light reflector to shoot this Assassin fly with its meal.
UPDATE: After years of using the same paper plates, its come to my attention that they will no longer be made. I have gone through many other plates and none compare to the original. All the new ones are either slightly glossy or made from recycled material and show grain or are slightly yellow in color. So, I have settled with a PVC material called Sintra. Its mainly used for signs but it works perfectly. Its a matte finish and completely white. An added bonus is it can be cleaned and used over and over again. And even better it comes in different colors like black.