Useless for Blender versions after 2.81, since a built-in denoiser in now in the software.

This addon brings the power of NVIDIA OptiX Denoiser directly into Blender 3D. It has 4 simple features:

Denoise render result
Render and denoise result
Denoise rendered animation
Render animation and denoise

It is built on top of a command line executable coded by Declan Russel (https://github.com/DeclanRussell/NvidiaAIDenoiser). The addon automates the usage of this executable to use it directly from Blender.

The two features of the addon that include rendering the image(s) before denoising disable any preview during the rendering process, and the UI will be frozen. This is why I recommend to render manually the image(s) and then click either "Denoise render result" for a single image, or "Denoise rendered animation".

When you denoise a single image, either from option 1 or 2, the normal noisy rendered image is keeped into the "Render result", and a temporary file is created and opened to let you view the denoised result. It is then easy to compare the noisy and the denoised images! This file is overwritten every time you denoise an image, no matter in which project. That is to say, if it is your final render, save it somewhere else.
When you denoise an animation, you must go in the folder where the frames are saved and view them here.

When using the addon for an animation, it is required that you specify where the image will be saved on your disk, for the denoiser requires to know where the images will be stored in order to read them. The folder where you save the animation must be different of the default "/tmp\", to avoid any conflict with existing images.
What's more, you must render your animation as an image sequence, not a video file, or the denoising won't work.

This addon provides an access to the OptiX AI Denoiser, and thus has the same advantages, but also the same weaknesses. For instance, it runs exclusively on NVIDIA GPUs. Note that due to the use of an external executable, it runs only on Windows. Also, denoising a very low sample image will result in strange results.

Works on both versions: 2.79 and 2.80 Beta (thanks to Roman Kornev (https://github.com/RomanKornev/))

DOWNLOAD LINK:https://drive.google.com/open?id=10WNZuG4oaMIyLn0Klay7_DpZw5GzYHGF

Tested on Windows 8.1 64 bits, with Blender 2.79.

Tested on Windows 10 x64 with Blender 2.80.0 Beta x64 (9149e894210)

Tested on Windows 10 x64 with Blender 2.79.6 x64

INSTALLATION

Download zip archive
Open Blender User Preferences
Add-ons
Install Add-on from File...
Select donwloaded blender_optix_denoiser.zip
Enable installed addon
Render panel now has OptiX Denoising section at the bottom

Developers, if you want to modify this addon, I recommend you to download the release directly, for the script will be the same as on this repository, and it will directly include the executable, which is not on the repository. The denoiser executable and all its dependencies must be in the "Denosier_v2.1" folder, which is placed in the same folder as the script, like this:

Denosier_v2.1/
- ...
icons/
- optix_logo.png
blender_optix_denoiser.py

No more in development, since Blender 2.81 has multiple embedded denoisers.