I bought this air conditioner filter by Duck and I might as well review it here. (note: check if Amazon links expire) 24 x 15 x 1/4 " and washable.
Before this, I used 3M Filtrete, which was very good, but turned black very fast and so was very expensive to upkeep. I also used this very sparse filter which took very long to change colour, meaning it didn't trap much. So I wanted a filter that worked well, but was washable.
So far, Duck is working very well. After about 2 weeks, the filter turns grey, with the pattern of my air con's grill.
A close-up of the filter:
When washing, it turns the water a satisfying grey. I just hope it holds up to washing, though. I saw a few pieces coming off, but that might be because the filter was new. Even if it doesn't last very long, it's the same price as Filtrete, which can only be used once.
Speaking of Filtrete, Filtrete gains 0.12 mg/cm2 when dirty.