BMW Cabin Air Filter
My BMW 745Li has filters on it's HVAC intake. Pretty nice, keeps the dust out of the car and, more importantly, keeps the evaporator clean. You should have seen the handful after handful of leafs and pine needles I pulled out of the evaporator on my truck. It doesn't have a filter.
Anyway you can buy two different types of filters. Regular paper filters ($35.50 for 2, the car needs two, one for the left and one for the right) or Activated Charcoal filters ($50.75). I always but the Charcoal ones, since I like the idea of strange smells being reduced before they come into the car. I thought it'd be interesting to cut open one of the old Charcoal one to see how it's made.
Pretty neat. Looks like they roll the filter paper in some kind of glue and then roll it again in Charcoal. Then put two pieces of of filter paper together in pleats to make the filter.