Well, it's not really much...but short of green screening a few layers of rain try this:

Create an instance of the LAVA media generator.

We'll work from top to bottom (left to right) in the properties panel for the LAVA generator. I'll do it in the order I did it...

Settings for the keyframe at the start (0 of the animation)

Length: 1 second
Noise style: Fractal turbulent
Number of Layers: 5
Frequency X: 30
Frequency Y: 30
Offset X: 0
Offset Y: 5
Progress: 0

Noise Parameters Min: 0.350
Noise Parameters Max: 0.620
Bias: -0.150
Amplitude: 0.830
Grain (amplitude fall off): 0.400

Colour A: Transparent R: 0 G: 0 B: 0 A: 0
Colour B: Light Grey R: 255 G: 255 B: 255 A: 255

To animate the rain, set the cursor to the end of the animation (the 1 second point). Change Offset Y to -5 and Progress to 10.

Play the animation (you'll want to make several copies (use the REFERENCE to the original file). Run them and voila...rain-ish looking something. Changing the start values of OFFSET Y and PROGRESS will change the speed and variety of the rain. Noodling with the min/max settings and other sliders will let you get as much or little rain as you want.

With this kind of effect, don't discount the value of a) people looking like they're getting rained on, or, more importantly, SFX of rain.

This kind of effect is like an accent...it doesn't have to be exact, it has to suggest rain.

To complete the effect, copy the layer and use finer lines (with lower Y offset and PROGRESS and drop a blur on them to suggest "layers" of rain and offer some perspective).

Hope that works.

I just tried this:

Add generated media - starry sky.
chromakey the black background out.
Use progress to make it move.

That adds a pretty good raindrop effect.

Unfortunately I haven't found a way to make the drops fall down yet.

Well done you lot! . .Excellent "settings" witten up . .will give it a try . .thanks for sharing . . we gotta get this Preset Manager thang working for people. . ?

Grazie

You don't happen to have Particle Illusion available, do you? Several such effects scattered through their library. Darryl

I know there are some veg files on the Vasst site for rain using generated media. Also if you have Pixelan spice filters, one of their film scratches settings looks like a very convincing rain actually.

Thanks to everyone for the suggestions. I don't have any of the plugins mentioned, but I will try out the "DIY" tips tomorrow.

Richard

Video some 'rain', add coloring to the water to make it show up more, could try a little milk. Just get someone up a ladder with a watering can and shoot the droplets against a blue sky, instant blue screen.
In the last of the Matrix trilogy they spent buckets (ha!) to get the rain to look right. They decided to have the sprinklers pointing up so the drops were fatter, except that meant raising the lighting grid a metre or two which knocked the light done a stop and so ....

Bob.

Well, I used the starry sky as suggested by nickle, but then modified it using some of the settings from anatavism's lava effect. I also set color B alpha to zero so that I didn't have to use chroma key after all. Also, setting the X freq high and the Y freq low and using Jagged Lines helps to suggest vertical streaks. Overall, it has turned out just fine.

Thanks again to all,

Richard

By the way, they are releasing a Massive Matrix box set this Christmas (at least here in the US). 10 DVDs total.