Paper most likely although it can be anything light colored, such as white cloth. The trick is to light it separately with flash so that it is just overexposed and so appears pure white on the image.

Steve

Edit: I forgot that I had written an article about it: http://www.backyardsilver.com/2012/02/isolating-a-subject-against-white-background/

White Plexiglas - Most home improvement stores.

The best is a white gloss laminate.
It exists different kind, less or more reflective.
You can find it in different colors, even black (shiny) very adapted for makeup and beauty products or luxury ones.
It is easy to clean and enough resistant to scratches (if you take a minimum of care).
Personally I buy it directly to the producer, it costs a lot of less, but you have to take the full piece 305 x 130 cm. As you cannot fold it it is difficult to store. You can roll it, or cut it using glass-cutter to make more little pieces.
http://www.google.com/search?num=50&safe=off&q=white%20gloss%20laminate&um=1&ie=UTF-8&hl=it&tbm=isch&source=og&sa=N&tab=wi&ei=zjy4Ua3VF4XJPMa4gbgE&sout=1

The problem with materials such as paper and painted materials is that they have a visible grain/texture.
This visible grain is more visible when you shot little objects and you are closer.
In the case of the white the grain/texture will disappear in the zones of higher lights, but it will be always visible in the shadows.

This does not happen with plexiglass or laminate.

Thanks everyone.  Great help as usual.  I already know how to isolate and white and was specifically trying to find out what the "shiny" stuff was.  Going to go investigate tile board as a result of this post.  Thanks again.