Unity Editor script with shorcut key (Shift + End) to snap a selected object to the surface beneath it, with full undo support

SnapToGround.cs

using UnityEditor;
using UnityEngine;
public class SnapToGround : Editor
{
	[MenuItem("Tools/Snap Selection To Ground #END")]
	public static void SnapToGroundNow()
	{
		foreach (Transform t in Selection.transforms)
		{
			Undo.RegisterCompleteObjectUndo(Selection.transforms, "SnapToGroundNow");
			MeshRenderer renderer = t.GetComponentInChildren<MeshRenderer>();
			if (renderer != null)
			{
				RaycastHit rayhit;
				if (Physics.Raycast(renderer.bounds.center, Vector3.down, out rayhit))
				{
					float offsetY = rayhit.point.y - renderer.bounds.min.y;
					t.position += new Vector3(0f, offsetY, 0f);
				}
			}
		}
	}
}

Source: https://answers.unity.com/questions/976328/editor-snap-to-floor.html