The eyedropper (pipette icon) allows you to sample from anywhere in the Blender window. The eyedropper can be used to select different kinds of data:

This is the most common usage.
Color Ramp
Dragging the cursor over the window to sample a line which is converted into a color-ramp.
This is used with object buttons such as parent, constraints or modifiers to select an object from the 3D View.
Camera Depth

Number buttons effecting distance can also use the eye-dropper.

This is used to set the camera’s depth of field so the depth chosen is in focus.

  • E will activate the eye-dropper while hovering over a button.
  • LMB dragging will mix the colors you drag over, which can help when sampling noisy imagery.
  • Spacebar resets and starts mixing the colors again.