Attached Link: http://www.renderosity.com/viewed.ez?galleryid=192650&Start=1&Artist=mboncher&ByArtist=Yes

Check out my picture "Surpise in the Fog" I I created rain by putting a cube in front of the camera to just past the tug boat, putting the rain texture on it to give me a deep feeling of rain. In Photoshop I only used the clone tool , smudge tool and blur tool to break up the predictability of the material and give it a more erratic feel. You probably could get away without the photoshop postwork though if you did your materials right. With some modification, you could probably make it snow.