+1 for the melamine boards, commonly sold as "shower board" or "tile board".

I bought $30s worth and covered 3 of my walls with it -works great as a cheap white board.

For any notation consisting of ASCII characters, I find that an emacs scratch buffer makes a better brain-extension than a board does. But when I'm thinking in terms of diagrams and math notation, having a chalkboard next to me (a real chalkboard) is a huge help.