The Rain.Water Shader is highly realistic shader specific for rainwater on surfaces. Because the shader consists of a few node-groups you can use them in your own shader or material. For example, if you have a PRB material with soil, then you can add these nodegroups to simulate rain and it’s water. It works already on a default plane and the result is stunning; it makes your scene suddenly much more realistic. Below an example.
I think it looks so realistic because it makes use of a wetmap. That’s what I never tried out so far, and it’s good I see it now. It’s a big, very big difference. Before, my water never looked that good. And that’s because there was no transition between water and surface, and now there is. If that is not enough, it animates straight away and there are also options for that. You can even turn off the circles or adjust them, or let the water flow in a direction. I know I sound maybe to over-enthusiastic, but for me a world opens. This is how you can get realistic animated wet surfaces like here, where it is raining.
To see how it works, you can do two things. What I did was just loading the RainWater v1.0.blend file and then open the Shader editor. If that is not your thing (I am usually quite impatient reading documentation), then best is to read the documentation on Blender Market.
The Rain.Water is not an addon but its a shader, or nodegroups to make the shader. You can append them every-time you want to use Rain.Water, but what I would do it save the node-groups in a node-group manager or save basic rain.water materials in an Asset Manager. To discuss how the nodegroups work in detail that is not that suitable for this article but again: Nodes and Noodles (the developer) has excellent documentation.
You can find Rain.Water on Blender Market.