Pedantically, they're known as dehydrator packets (at least here in the U.S.)
Technically, they are, as Mamyalynne intimates, Silica gel, which is a desiccant, a substance that absorbs moisture. Here we see them in small clear packets with a small nubbin of something that turns pink in color if moisture is actaully absorbed.
Being, on the surface, one of those rather annoying little things in modern life, they actually have several other uses... such as:
Throw a couple in your filing cabinet to keep important papers dry;
I sue them in my garage tool box to help keep tools from rusting;
Mrs. C found that they help in drying flowers. She puts several packets in a brown paper bag and adds the flowers;
We aslo add one or two in our bags of opened potato chips (think ya'll call them "crisps, no?). Keeps the chips nice and crispy...