Based upon plant research by entities like NASA, Air Purifier Review has assembled the 15 houseplants that are best at cleaning your air (without electricity). The overall best? The Areca palm, also known as Chrysalidocarpus lutescens. [AirPurifierReview via Unplggd]
I've always assumed that air purifiers (at least the high end ones) work by removing dust and micro-organisms from the air. Some of the ionic ones can even help with smells.
I would imagine plants are good at getting rid of smells and a few other contaminants, but I don't think they're going to do anything about dust and micro organisms.
Quick side note, plants require light, and letting more light into your home will help kill off a lot of micro organisms that are floating around or on your furniture. So it's win-win if you consider the plant is not directly doing the work.