They are my friend. We use them for science projects, art projects, lists for school, a different way to work math problems or to enhance our religion lessons.
Some people call them white boards, or dry erase boards -- I refer to them as wipe-off boards (I doubt I'm supposed to hyphenate them, but I do).
At Lowe's you can buy 'white panel board', 'shower board' (if you call them or stop by and tell them that you're going to use it with dry erase markers they'll know). It's about $10 for a 4'x8' sheet! My Lowe's will make 6 cuts for free, so they'll slice it down into the sizes you need. I have 3 large wipe-off boards for $10 and we use them ALL THE TIME! The kids like them for pictionary.
For more advice you can read through http://digg.com/mods/DIY_Dry_Erase_Board
I thought this idea of using a foil for a wipe-off for the kitchen was interesting, too!
My FAVORITE thing to use them for is Chalk Talks from the Faith & Life books!!