Product review (2 stars): 20 x 20 Microfiber box fan filters
These didn't work very well for me.  I had them on my fan all last summer, and in the end, there was a slight brown color.

When I took it off and ran the fan, it really blew out cold air with a great force!  Something I hadn't had all summer because of the filters.  With the filter on the fan, it didn't blow very cold air and the force of the air wasn't great.  I wouldn't buy these ever again.