I have published an experimental release with support for Nvidia’s Optix denoiser in the viewport.
The denoiser is started once the viewport render is paused. It denoises in the background without blocking Blender’s interface.
Download link on the release page:
Here is a video showing the feature in action:
This release only runs on Windows. The Optix denoiser requires an Nvidia GPU.
Note that I do not guarantee that this experimental branch makes it into the official releases.