almost free garage heat – just drink a lot of soda | Hemmings Daily
Well, not a soda drinker, but can find empty cans being thrown away as garbage, or ask people you know to give you the extra cans.
This would be a fun project, but need to read over and over, to figure how to get the heat INSIDE.
Also a great way to keep some feral and/or stray cats warm for the winter.
Be sure to read the comments on that site, as some of the folks have come up with some more ideas.