Originally known as luftdaten.info, airrohr is the German project on which Air Apparent is based. The project started as a collaboration with the Future City Lab at the University of Stuttgart and has now spread across the globe with over 1,200 active sensors worldwide. You can follow them on Twitter or visit their website.
Smart Air is a small social enterprise that promotes DIY air purifiers as a low-cost solution to indoor particulate air pollution. They back this up with fascinating experiments on their blog looking into a wide range of air quality questions and finding answers for themselves.