Sent to you by Chris Hunter via Google Reader:
When replacing the air filters in your home, add a few drops of vanilla extract—or another scented extract of your preference—to eliminate odors and keep your home smelling fresh.
You won't want to saturate the filter. Just add enough to give the air that passes through it a nice smell. If you don't have central air, you can accomplish something similar by putting some extract in a coffee mug and placing it in an oven for an hour.
How to Remove Musty Odors from Your Home | Today's Home Owner