While you're cleaning out your gutters and changing the batteries in the smoke detectors, don't forget your furnace air filter.
This one was installed last fall. I use Honeywell filters that are pleated and about 4" thick. Pretty nasty looking.
The back side is nice and clean. This is what the new filters look like (except for the wire mesh which is only on the back side). The 4" thickness along with the pleating increase the surface area tremendously compared to a generic 1" filter. Because of the extra surface area I only have to change the filter twice a year.