Is Nik just a plug in or can it be used as a stand alone?
It can be used stand-alone like all the other Nik apps.
Yes, any NIK plugin can be used as a standalone - you can run them from here c:\Program Files\DxO\Nik Collection\ or by right click on the image and select with “Open with” functionality.
but as a standalone you cannot open a PSD file, only JPG: right?
Yes, you are right - PSD format is not supported.
well, it will be nice to be able to open PSD files with the standalone versions. Should I open a feature request?
As you wish
If you think it’s important for you and other users will support your idea, then go!