Color Checker Passport has been in my goodies bag since day one. I don't always use it, but there are times where it is perfect for the job.
This tool is about the size of one of those old school check books. I use mine to set custom white balances and preserve color profiles. Simply take a shot of the passport and use it later on in your editing process. It has a plugin for Lightroom. You can export a DNG profile to use for perfect color everytime. With the click of a button you can gently warm or cool your image by selecting subtle changes in the grey scale. It is a must have for real estate, product, and even portrait sessions. It will save you a lot of time when you are trying to correct your color profile.
Meter the grey card to set your exposure in your current lighting conditions. Shoot the other side for the full color profile image to preset the color profile for your shoot. If you have to have precise color in your images then you have to have one of these in your bag.
One thing to keep in mind with this item is that it is printed with pigment instead of ink. You do not want to touch any of the printed areas. The oil from your skin can damage the color of the card.
Check out this quick video demonstrating how easy this item is to use.
Want to do some more research? Here is a link to one of these on Amazon.