When you buy an AeroPress it comes with a pack of paper micro-filters. These work fine and are easily disposable. However, for something different, there is this stainless steel AeroPress disk. The disk is reusable and has ultra-micro perforations so that only water gets through the filter, even when brewing a super-fine espresso grind. Many people prefer the flavor that stainless steel filters allow as opposed to the bleached paper filters. Paper filters absorb some of the oils from the coffee beans, metal filters however allow those oils to pass through into the cup of coffee. For those who like to brew dark roasts (which is where the AeroPress shines since it is, in a way, a manual espresso maker) the metal filter may provide a richer tasting cup of coffee.

(Thanks Nathan.)