Others have already answered this and I am sure it is in other topics as well.
However foil can be used. I would only use it for CFL grows though or in an emergency. It does create hot spots and can burn your plants. We don't want that now do we.
You can use the emergency blankets made from mylar, buy a roll of mylar (not overly expensive), flat-white paint as others have stated, Chrome vinyl I hear also works well (never used it but heard it works) any info on this would be great as I have a few rolls of it laying around, There are also reflective insulator blankets that have a diamond pattern on them which can also be used. I think they sell rolls of this now for growers. I also know that some grow tents come with this type of reflective lining.
I have herd of people using a few other things as well but I can not remember exactly what it was they used.
-Ammastor-