ColorCube is a simple application that lets you load an image and displays the color space of the image in a 3D cube. You can rotate the cube with a slider.

It has very few options since it really doesn't need much. A neat feature is the ability to only show colors used more than a certain number of times, set by the Density Cutoff spinner. So, if Density Cutoff were 15, then you would only see colors in the cloud that were used on at least 15 pixels.

You can also specify the cloud opacity, which is often useful for seeing the 'inner' structure without raising the density cutoff.