You can’t always find a blank wall of the proper color when you need one, so a portable white-balance device would seem to be useful to any photographer who’s graduated to an SLR, and one that takes up no extra space in the bag would seem to be even more so. Behold the White Balance Lens Cap:
Put your camera in White Balance mode and snap a picture with the lens cap still on the camera. The camera does the rest of the work, adjusting the white balance to get true to life colors in your pictures. Especially helpful if your camera allows for white balance adjustment but does not take pictures in RAW format.
The handy lens cap replaces your old one, takes no additional space and does not get lost in your camera bag.
Available in several lens sizes from Photojojo; prices vary with size.