There is a veg file on the sundance site that is quite convincing of rain, including the splat on the ground. Still isn't the same as shooting with colored water, but will work for short scenes in many instances.

Colored water?

I'm gathering that your footage has been shot, so you have to do this in post ? If so, try shooting some lit "rain" (spray from garden hose or similar) infront of a black velvet drop, then comping in luminesence (sic) mode over the top.

A lot of the time in Hollywood, to make the rain show up on print, water wastinged with white is used to create the look of thick rain. I have a link to this on my desktop, but I'm traveling...and can't find it on Google. This isn't so much used any more, not from what I've seen. I did see a production of Singin' in the Rain that used a milk colored rain on a live stage a couple years back. I don't know what they used.

For the film version they had to use a lot of side lighting to get the rain to show up, which made lighting the whole scene more of a challenge.

There's a great 3D program called Impact Plus 5 from a small English company named Serif. It has particle effects for creating rain, snow, smoke, fire, etc. I haven't tried rain, but it might do what you want. You can adjust the particle color, transparency, lighting, source, size - virtually everything about the particles. In addition it makes very good extruded 3D titles with motion and will export in uncompressed AVI format.