Best cheap photo accessories: Reflector
A reflector is one of the simplest but most effective accessories you can buy.
You use it to balance exposures by bouncing light into shadow areas by positioning the reflector in front of the subject at an angle.
You can make your own simple reflector out of a piece of white card.
This will be fine for small subjects, but for larger subjects such as portraits, a folding reflector such as those from Lastolite are much more convenient to carry and use.