If your car has developed a bit of a funk (or if it was previously owned by a smoker), use a couple of bricks of lump charcoal (pre-BBQ) to help get the odors out. Place the charcoal in a paper bag or wrap it in a piece of scrap fabric, and stash it under the seats. After a few weeks you should notice that the smell has faded.
What other new uses do you have for charcoal?
[photo by Ted Morrison for This Old House]