Nothing fancy is needed. You can use a white card or a gray card. Since almost everyone has a gray card, that's the easiest thing to use and you know that its a neutral color. Often time you're not sure about white. Since I always carry a black/gray/white target, I just use the middle strip of it. Here is what it looks like:
Re: What are you guys using for white balance?
I also use it for zeroing in on precise exposure. In the example photo above of Dee, I've drawn a dashed line around the area that you would usually fill the view finder with. If you have proper exposure, then you'll see three spikes like those shown on the camera's histogram. You use the same image for the CWB balance reference shot.
If you're shooting RAW, where you can choose the White balance later, then you can use the reference shot with the RAW eyedropper for your initial WB. If shooting JPG, then you can use the reference shot with Curves and its three eyedroppers for color balancing.
The target I use is one of those that folds up and is small enough to put in my back pocket.