Insulated Window Covers
I have some cheap ($15-20) sleeping bags hung over my windows to provide insulation now during the colder Winter months. I've done a rather poor job of making sure they are kept tight and prevent leaks. Still they do help quite a bit. Peeking under one while one of the cats is in the window sill I can definitely feel the temperature difference.
Over on the Permies forum, they are having a much more detailed discussion on how to make and use such insulated covers.
Good thread for those who can sew or are looking for more in depth information.