With the Node Expressions addon for Blender 2.8 you create node groups by entering a formula. In case you are not good at math, this addon encourage you to use a bit more math in your node setups. So in case you want to dive into a bit more sophisticated node setups this might be interesting for you. Also for those who are good at math this addon a welcome contribution. There is a free version and a paid version. You can do already a lot with the free version and the paid version has a reasonable price.
Even though I am able to code an addon for Blender in Python, I am not good at math. And for that reason this addon caught my eye. The developer provides a good documentation and examples.
How the addon works.
After installation you will find the addon in the shader editor (previous called: node editor). Press Shift + A and then you will see “Maths Expression” at the bottom. Click on Maths Expression and you can enter a formula. For now a very simple example: Average = (a+b+c)/3. This will create a node-group as shown below.
You see that we have three inputs (you could plug the fac of a texture in here) and as output you see Average. When we open this Node Group we see as shown in screenshot below.
Now, here an example of how to use it in a shader:
I am not sure if you can see it, but I this case I plugged the Average output into a color mix node (well, used screen actually) and also feed it into a Bump.
Full documentation and download
This is just a very simple example, but there is way more. For a complete documentation I suggest you to visit: BaldingWizard or Node Expressions addon on Github or Nodes Expressions addon on BlenderMarket. You will find the downloads there as well.
Downloading is pretty simple and as usual. Download the .zip file. And in Blender you go to Edit > Preferences > Addons > Install. Then browse to the .zip file you just downloaded and press Install, save preferences and refresh.