Well, if you rent or are in an older home and just can't do anything about new windows, this may be a short-term fix. They can help insulate the window, but I have issues with the no-light thing. However, when my fiance works midnights and all the light shining through the window mid-day affects his sleep...well, this just means...Christmas! He has complained about the brightness for years now, so, no more.

The material isn't so eco-friendly, but the potential energy-savings is worth mentioning.