According to a study performed by NASA, as noted in the book "Naturally Clean" by Seventh Generation founder Jefferey Hollander, the top five plants for improving indoor air are the Boston fern, Areca palm, Lady palm, Bamboo palm, and Rubber plant, in this order. I recently added some Boston ferns to my apartment and the air quality drastically improved, and much more than provided by the Rubber plants that I already had. Plants add negative ions to the air, which improve air quality, reduce dust, and improve your sense of well being and your health.
This article here, although a bit more scientific than general reading, provides excellent information about improving indoor air with indoor plants.
In my experience so far, two of the above plants in every room substantially increases the quality of indoor air. The challenge is to place the plants effectively to avoid cluttering your home.