Plugin that lets user select focal point/cropped area
With Craft CMS 3, focal points are built into the Image Editor.


  To launch the Image Editor, double-click on an image (either on the Assets page or from an Assets field) and click on the “Edit” button in the top-right of the image preview area in the HUD. Alternatively, you can select an asset on the Assets page and then choose “Edit image” from the task menu (gear icon).


more info 



Here's how you could use the asset's focal point in your code:

Check if image has a user-defined focal point:
{% if image.hasFocalPoint %}

Get the focal point values:
{{ image.focalPoint.x }} / {{ image.focalPoint.y }}

Get the focal point values as CSS (e.g. for use in the object-position value):
{{ image.getFocalPoint(asCss=true) }}




I think in Craft CMS 2, these two plugins were widely used:

Focal Point Field
Focus Point