With many modifiers, you can select a target object with an eyedropper (e.g. boolean, mirror, array).
Currently you can only select objects in the viewport. I think you should be able to select objects in the outliner as well. Seems like it’d be a very easy implementation?
Thanks for considering it!
(Sorry if already mentioned.)
[I have crossposted this here from the paper cuts mega-thread per the suggestion from Pablo]
Currently, user adds a modifier or a constraint and then, his mouse pointer goes from Modifier/Constraint tab to Outliner to start Drag&Drop and goes back to Modifier/Constraint tab to end Drag&Drop
By using eye dropper, there will be only one mouse movement.
Most efficient way is probably to select target and carrier of constraint/modifier and use shortcut to add it.
But that is not the most discoverable one.