We generally just use Yahoo to check the local temperature when we need it. But at home, we love having our little window thermometer. Here's a really nice little stainless steel dial thermometer from Yardiac. The dial is only 4 3/4" diameter and it can be mounted at your window.
Ours is just a cheap little one left behind by the former owners of our apartment. It's mounted just outside our window. We're glad they left it or we never would have thought to put one up for ourselves.