Although it looks like something you'd hang from your rearview mirror next to the fuzzy dice, Datacolor's SpyderCube is actually an ingenious tool for maximizing the dynamic range and obtaining neutral white balance for your digital photographs. Datacolor describes the SpyderCube as "the first all-in-one raw calibration device for fast and accurate whitepoint correction as well as bracketed adjustments for highlights and blacks," which I find a bit misleading, but it nonetheless seems as if it has the potential to aid those of us who batch process large numbers of images shot under consistent but uncontrolled lighting.
The term I take issue with is "calibration device," because it makes it sound as if you're somehow optimizing the camera's behavior by using it. You're not; you're optimizing your processing of the resulting image files. You photograph it to define a reference white point, black point, and various other characteristic points that you then use to more accurately and consistently retouch photos--or create a profile based on it for batch processing of the photos--shot under those lighting conditions. In fact, I suspect if you tried to use the entire cube for setting manual white balance in the camera, the closest a camera offers to calibration, it would send too much data and confuse the system (I haven't yet tried it; this is based on my discussion with the company when the product was just a spec.)
That said, it seems far more useful than a simple white/gray reference point. Since it's three dimensional, it picks up illumination characteristics of the scene that a flat card can't; the addition of a black point gives you the ends of the range over which to set your levels adjustments and the various gray chips provide midtone points.
For $59, it might be a useful little device to toss in your camera bag. It's scheduled to enter our dimension in March.