Ever since Blender made the switch to the Cycles renderer back in 2011, its shader nodes have lacked an interactive preview for each node. I spent the first months of 2021 writing an addon that adds this functionality: Node Preview, which is available on the Blender Market.
It utilizes a Blender instance started in a background process to render the thumbnails without causing stutter or lags in the main Blender window – the UI stays fluid and responsive at all times. All Cycles nodes are supported and it can also be used with Eevee, since most nodes are shared between the two renderers. Only Eevee-exclusive nodes like ShaderToRGB and Specular are not supported.
The addon currently runs on Windows and Linux, and a Mac OS version is planned. Since it contains a compiled C++/Python module, only particular Python versions are supported – currently, this is Python 3.7, which means the addon is compatible with Blender 2.80 up to 2.92.
In Blender 2.93 beta the Python version was upgraded. This makes a re-compilation and future update of Node Preview necessary, which will happen once 2.93 is out of beta.