Tripod Mounted D700
Nikon 50mm 1.8 Lens
Nikon SB-600 Speedlight in Umbrella
2 Second Exposure @ f/8
For this shot I set my D700 up inside the house, facing outward. I placed my SB-600 unit up inside the metal frame of an umbrella and the sensor on the front of the speedlight secured it within that framework so that I didn't have to tie it down each time. This was a big deal because I would have had to find string or something similar around the house and who has time for that?
The flash was set to TTL with a negative EV dialed in as to not overpower the shot with blown areas, and I began with the camera set to about 1/125th of a second. This produced some nice shots but the rain was frozen as round droplets and it didn't give the desired effect. So, I began to slow the shutter down to draw out the droplets into streaks as you see above.
Now, the flash is what freezes the action and not the shutter, in this instance. You might think that slowing the shutter speed down to 2 seconds wouldn't have an impact on the look of the photo because the flash should still do its thing for the fraction of a second it's firing. My guess is that the Nikon CLS was maintaining communication between the camera and the flash and when I slowed the shutter down on-camera, the camera then told the flash to extend accordingly, providing a longer period of time during which the drops could be seen.
I grabbed some glasses and a book from the shelf to add some weirdness, and to ensure that my wife called me a 'big dork,' and was good to go...