Long exposure photography is probably my favorite style; indeed most of my better pics were clicked when I had an awe-inspiring vista and ample time on my hands.
Over at 500px, Matt Kloskowski has penned a good tutorial on this. Although I would have preferred a bit more content about the actual on-camera photography instead of on post-processing in Photoshop, its still a very good read. The example he has used truly shows the ethereal beauty of flowing water and other landscapes shot with a long exposure.
People were putting 3, 6 or even 10-stop neutral density filters on their cameras to slow shutter speeds. The result was this beautiful long exposure photo where clouds, water and lights had a beautiful streaky motion-like feeling to them. Personally, I love photographing the water. Beaches, waterfalls, lakes, streams, you name it. But I'm not a huge fan of choppy water. I love reflections and smooth silky water. So for me, long exposure photography was the answer.
I'd like to add my two cents here and say that long exposure photography is great in low light too - while some photographers like to increase ISO to manageable levels and then choose the fastest shutter speed, more often than not I drop the ISO to 100 and increase the shutter speed to the highest I can. Besides getting rid of the noise, the long exposure ensures that the ambient lighting and atmosphere really permeates the image.
In the three pictures below the environment was completely different and this comes out beautifully.
In this picture of a city skyline, the ambient lighting is primarily from the electric lighting in the city; the heavy atmosphere also has made the picture slightly soft.
In this picture of the canyon under the starlit sky, ambient lighting is from the stars themselves and the image is very crisp owing to the clean atmosphere.
The third one of the boat on a beach was lit with distant fluorescent and Sodium-vapour street lamps, while the boat itself is sharp the windy conditions have fuzzed out all other details.
Sometimes I also close the aperture down to f/22 or more which necessitates a long exposure shot just to get starbursts around the light sources. Here is one taken late in the evening.
Matt concludes by saying...
As an outdoor shooter, I think it offers you a whole new area of creativity, because we're typically so used to shooting static scenes with no movement. For me, long exposures bring the best of both worlds together.
...and I couldn't agree more.