Released. See Spirit Level Photo Effect
OK there are several methods to rotate an image in Paint.NET. But the tool I missed always is the the spirit level.
This tool works in the way that you put your spirit level (represented by a line) on the image and the image will be rotated so that the spirit level is horizontal.
The Api does not allow us to add tools so the solution is using a tools effect.
So just to see if you understand and accept the UI here is
Near to release a version for people who like to add/complete a translation