(Via SurvivalBlog and Timebomb2000)
Mother Earth News has a useful article on build-it-yourself solar collectors for your windows.
I made a couple of highly simplified solar collectors for my outbuilding some time ago, and they could raise a 21' x 21' x 8' mostly uninsulated building 10-15o on a winter's day. This was with about 6 hours of direct sun with an ambient temperature in the 30s, and without the collectors being angled as they specify. I suspect that better-built and installed collectors would perform much better.