If you enjoy being nice to the environment (or if you're just looking to save a few bucks), building a solar heater is a useful weekend project that costs very little, but can heat a good portion of your house with ease.
The basic principle behind this air collector is the same principle that makes your parked car hotter than the outside temperature — sunlight shines through the glass on the collector, converts to heat, and then as such can no longer escape out the way it came — instead collecting on the inside (which can then be vented into the house on which it is affixed). This particular hot air collector was made out of some used 2 x 4 metal light fixtures, some old panes of glass, and black paint — things that you can probably get for free if you know where to look (or happen to have some around that are losing life). You'll also need a bit of aluminium tape, silicone caulking, and a few tools to put it all together.
They're not necessarily the prettiest, but the more you make, the more heat you can you can collect and pump back into your house — and there's bound to be a side of your house that no one really sees anyways. Hit the link for the full instructions.
Almost Free Solar Hot Air Collector [Instructables]