[UPDATE: thanks to Zephyris, which has reminded me with his good Instructable
that you first have to reverse one of the two CPL filters, of course I've found his Instructable shortly after publish mine, as often happens to me, and in this case I've also forgotten I already commented his instructable two years ago...]
A Neutral Density
(ND) filter is a cool gadget, you should always carry it (at least daytime). You can find easily ND2, ND4, ND8 filters, these would darken your scene of 1, 2 or 3 stops respectively. When daylight is very strong it's difficult to keep a very large aperture, so you couldn't have low deep of field or nice bokeh. In these cases an ND filter should be very useful. I also use it to keep very long exposures in daytime. You can obtain unique effects, with cars and people moving as ghosts.
Of course you'll reach a better quality with traditional ND filters, but if you have two polarized filters
you can experiment a really funny phenomenon. I suggest circular polarized filters (CPL) rather than linear ones, because with CPL you can keep automatic focus of your camera, whichever is the rotation angle of the filter on the lens. You can buy CPL filters on eBay
, or at your favourite photography shop, you can find every diameter.