Taking long exposures is one of my favorite techniques. I don't go anywhere without my B+W 1.7 and 3.0 filters and being able to slow things down, even in full sunlight in the middle of the day, can help create some unique situations.
 
But sometimes the long exposures make it difficult to use GND filters, so here's a simple digital technique to simulate them.

I recently attended the Calvert Photography Club's November photo trip to Great Falls. We visited both the Virginia and Maryland sides of the park on the same morning. Being only 3 days after hurricane Sandy departed the Washington, DC area, I expected the water levels to be much higher than they were. However, you could definitely tell there had been a jump in river flow. It was much dirtier than usual and the water level was a little high (much higher than it had been over the majority of the summer). It was here I took a series of burst shots of the waterfalls and put them in motion.