While beta testing the Mac version of HitFilm 2 Ultimate, I decided to try create the effect of rain hitting a window. So I sprayed my window with a garden hose, took a photo of the drops of water, and created some particle textures. Then I applied them to a couple of particle systems, and created this video:http://youtu.be/DQHfXTKbNrQ
I used some random footage from a few years back, and I realize that its pretty obvious that I'm not getting hit by any raindrops, nor is the surface of the water in the BG. But the illusion that there is a window between the camera and the subject seems to work pretty effectively.