In darktable, selective editing was a long awaited feature. Our development version now allow limiting module effects to a region of the image.
Remember the old times, the red light of the darkroom, the smell of the developing bath …
Remember when you were using your hands or a small piece of cardboard to achieve some masking …
Now you can do that in darktable.
example
let take this photo as an example.
This is a companion discussion topic for the original entry at https://www.darktable.org/2013/04/masks/