Clean Air Plants

An 1980s NASA report suggest houseplants are excellent air cleaners.

Besides absorbing carbon dioxide (which might be an issue during cold seasons when there is little indoor air exchange) and releasing oxygen, some plants also absorb toxins (ammonia, benzene, formaldehyde, trichloroethylene, and xylene).

This article provides a list of plants that NASA has called "nature's life support system." A more detailed list can be found on Wikipedia.