A Unity editor tool mode (like Translate, Rotate, etc) that lets you click/drag to position objects in the scene.

PlacerTool.cs

// Author: John Austin
// Licensed under MIT License

using System;
using System.Reflection;
using UnityEditor;
using UnityEditor.EditorTools;
using UnityEditor.ShortcutManagement;
using UnityEngine;

// ReSharper disable once CheckNamespace
namespace ClockworkEditor
{
    /// <summary>
    ///     An editor tool mode (like Translate, Rotate, etc) that lets you click/drag to position objects in the scene.
    /// </summary>
    /// <remarks>Press and hold shift, then click and drag to place the object in the scene.</remarks>
    [EditorTool("Placer Tool")]
    public class PlacerTool : EditorTool
    {
        private bool shiftHeld;
        private Vector3 mouseDownWorldPosition;
        private Vector3 mouseDragWorldPosition;
        private Vector3 mouseDownWorldNormal;
        private Plane mouseDownSurface;
        private Vector3 draggedForwardVector;
        private Quaternion orient;

        // This is called for each window that your tool is active in. Put the functionality of your tool here.
        public override void OnToolGUI(EditorWindow window)
        {
            if (shiftHeld)
            {
                // Forces this control to take priority and all other handles to be disabled.
                // We only take control on shift, so you can pick-select objects when it's unheld.
                HandleUtility.AddControl(0, 0);
            }

            Event e = Event.current;
            switch (e.type)
            {
                case EventType.KeyDown:
                    if (e.keyCode == KeyCode.LeftShift)
                    {
                        shiftHeld = true;
                    }

                    break;
                case EventType.KeyUp:
                    if (e.keyCode == KeyCode.LeftShift)
                    {
                        shiftHeld = false;
                    }

                    break;
                case EventType.MouseDown:
                    if (e.button == 1)
                    {
                        return;
                    }

                    if (e.button == 0 && shiftHeld)
                    {
                        
                        // A little hacky, but this is the only reliable way to ray cast into the scene without needing
                        // colliders on all objects.
                        
                        object[] args = {Event.current.mousePosition, null};
                        // ReSharper disable PossibleNullReferenceException
                        Type type = Type.GetType(
                            "UnityEditor.SceneViewMotion, UnityEditor.CoreModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null");
                        MethodInfo methodInfo =
                            type.GetMethod("RaycastWorld", BindingFlags.Static | BindingFlags.NonPublic);
                        bool didHit = (bool) methodInfo.Invoke(null, args);
                        // ReSharper enable PossibleNullReferenceException

                        if (didHit)
                        {
                            RaycastHit h = (RaycastHit) args[1];
                            mouseDownWorldPosition = h.point;
                            mouseDownWorldNormal = h.normal;
                            mouseDownSurface = new Plane(h.normal, h.point);

                            Undo.RecordObject(Selection.activeGameObject.transform,
                                $"Place Object ({Selection.activeGameObject.name})");
                            Selection.activeGameObject.transform.position = mouseDownWorldPosition;
                        }
                    }

                    break;
                case EventType.MouseDrag:
                    if (e.button == 1)
                    {
                        return;
                    }

                    Ray ray = HandleUtility.GUIPointToWorldRay(Event.current.mousePosition);
                    bool hit = mouseDownSurface.Raycast(ray, out float enter);
                    if (hit)
                    {
                        mouseDragWorldPosition = ray.GetPoint(enter);
                    }
                    else
                    {
                        mouseDragWorldPosition =
                            mouseDownSurface.ClosestPointOnPlane(ray.origin + ray.direction * 1000);
                    }

                    draggedForwardVector = (mouseDragWorldPosition - mouseDownWorldPosition).normalized;
                    orient = Quaternion.LookRotation(draggedForwardVector, mouseDownWorldNormal);

                    Undo.RecordObject(Selection.activeGameObject.transform,
                        $"Place Object ({Selection.activeGameObject.name})");
                    Selection.activeGameObject.transform.rotation = orient;

                    break;
            }

            Handles.DotHandleCap(0, mouseDownWorldPosition, Quaternion.identity,
                HandleUtility.GetHandleSize(mouseDownWorldPosition) * .05f,
                EventType.Repaint);
            Handles.color = Color.white;
            Handles.DrawLine(mouseDownWorldPosition, mouseDragWorldPosition, 1.5f);
            Handles.color = Color.blue;
            Handles.DrawLine(mouseDownWorldPosition, mouseDownWorldPosition + draggedForwardVector * .2f, 3);
            Handles.color = Color.white;
        }

        [Shortcut("Clockwork/Place Object Tool", KeyCode.Y)]
        private static void Shortcut()
        {
            ToolManager.SetActiveTool(typeof(PlacerTool));
        }
    }
}