I use reflectors quite a lot. The possibilities are endless. On location they can be used to get a light source to somewhere that you couldn't otherwise get one, because they are so thin and light. For example, if I want an overhead light and don't have enough height for a boom and light I just bounce a spot from a reflector held against the ceiling. Similarly if the space is so compact that even the furthest light position would result in too much square-law falloff, I 'fold' the light path with a reflector.
Additions to the reflector, such as the aluminium foil already mentioned by Ben, can be used to dapple the light a little, and introduce subtle variations in colour. Here again the possibilies are endless. Visit a fabric store and pick out gauzes etc that can be draped over or clipped to the reflector, not necessarily covering the whole reflector. Golden colours for a warm effect, blues and greens for cold. I go for very subtle, barely perceptible, changes but others use them more dramatically. More difficult to describe than to demonstrate.
Best,
Helen