To catch sight of a rainbow, you usually need to weather a storm. Unless, that is, you have the Solar-Powered RainbowMaker, designed by David Dear at Kikkerland. It's a little gadget that does exactly what it sounds like.
The device is solar powered in two ways: the small panel uses sunlight to power the rotation of Swarovski Crystal, which in turn refracts sunlight, creating rainbows. The steady rotation keeps the rainbow moving, creating, as Kikkerland notes, great entertainment for cats.
You could take a DIY approach that would circumvent the extra plastic by simply hanging a refracting crystal in your window, though it's unlikely to turn quite so nicely.
Hat tip: igreenspot