I just posted
this as a response on Megatokyo's board...figured it was valuable
enough to post here as an effect tutorial.
Have a source pic
you got something to rain on...not really a step 1 but I had to
Make some noise
Create a new layer above the background layer and fill it
Run Filter>Noise>Add Noise
Choose: Gaussian, Monochrome and an amount of around 65..depends
on how big your pic is and how much rain you want.
a motion blur filter
Use a setting similar to what I have here.
Change the layers blending mode to "Screen"
(That makes all the black transparent)
Now do this:
Slide the central grey arrow to the right till the rain looks
Now you have
Add to it's realism by adding puddles and reflections on hard
surfaces..make little white "v"s where the rain bounces
off stuff in the foreground.