I found something interesting at the local mega-discount store...cardboard coils with scented gel attached that you clip to your system's air filter. When the air is blowing on the filter, the scent filters through the system and your house. I bought the cinnamon scented one and it's actually very nice!
My question is, does anyone know if these things are harmful to the HVAC system at all? It's small enough that it won't block air flow, so I'm more concerned about the chemical aspects of it. I wouldn't think it was very harmful, but ya never know.