Three humped camel???

I have a kodak grey card, onto which I've painted one matte black stripe, and one matte white stripe. (approx in thirds).
Keep it in my camera bag as a quick reference tool.
when exposure on camera is set right, I get one hump at left, one at right, and one in middle, of the histogram on my camera.
Even though I have lightmeters, i find this card useful as it also allows me to color balance and set black and white points in computer later.

Hint: you can buy cheap ready made grey cards from www.qpcard.com

The dyes they use are flat spectrum. They don't suffer from metamerism (where the colour appears to be different under varying lighting conditions). This is also important in a decent grey card because you want it to be true grey under all lighting conditions (read this link for more info: http://www.extremetech.com/article2/0,3973,13764,00.asp )

I have the QP201 which has colour patches on too, and you can use their software to correct JPGs and TIFFs. Quite nifty, and not very expensive either

Matt