I use the Bluair 601. They are ridiculuosly expensive (about $600 new) but i snagged one on craigslist for less than $200. It has 3 fan levels and it is very quiet unless it is in full blast. It works very well in reducing allergens and odors at home. I like it but the filters are pretty expensive, about $80 . Do a google search on blueair and the reviews seem very positive.
What you want to avoid are the Ionic Breeze purifiers which do not have the capacity to move huge volumes of air because of the lack of fan.