Neutral Density (ND) filters are a great tool for producing unique photos in lighting conditions where a long exposure would otherwise not be possible. This 7-minute video tutorial goes in to detail about which filter to select for your scene, with before and after photos showing the effects possible.
He then runs through the use of a 3-stop ND filter for shooting waterfalls, 6-stop ND filters for creative city shots, 10-stop NDs for slowing daytime exposures to the 1-minute mark, and the ultra dark 15-stop ND for long 3-4 minute exposures on cloudy days.
Take a look at Craig’s video at the top for more detail on how to use these filters, and for some inspiration for your next shoot. If you like what you see, be sure to check out the archive of videos on his YouTube channel.