If your fridge has a really smooth, glossy finish, you can just write directly on it with a dry-erase marker. In my experience, it doesn't wipe off quite as easily as a "real" whiteboard, but a little Windex took care of it immediately. We used to do this at a bakery I worked at to keep inventory on some refrigerated items.
My problem is that my beer fridge has a texture on the door - to do anything like this, I would have to fill and sand the door, then repaint it. I think I may just go with a magnetic whiteboard.
Argh, beaten while I typed.