Gradient eyedropper proved to be one of the fun features for presenting Windows Presentation Foundation/Expression Blend. While fun for presenting, its ability to capture the color spectrum of the mouse-crossed area is useful in real-time designing too.
Try this for example... Start up your Blend, create a new project and put a rectangle on a window. Select Gradient brush for rectangle's Fill property and click-select the Gradient eyedropper tool (next to the gradient/color selector):
With eyedropper selected, drag a mouse, holding the left button, vertically across the rainbow color scale (part of the color selector). And here comes the rainbow rectangle...
You can color swipe any area of the screen, as long as your eyedropper cursor stays inside Blend's main window. This is somehow different to using the "regular" Color eyedropper tools, which allows you to capture even colors outside Blend's bounds.
Now go color your life ;)
On a side note: Expression Blend 2 December Preview is out.