Moving GameObjects with Mouse

I've used this one to move a crosshair in a 2.5D game.

It's a bit more complex, but it has two important things in it

1. It correctly accounts for the offset (depending on where you touch, other scripts will place the object below your mouse cursor), so that the object is moved correctly instead of it's origin being placed below the mouse cursor

2. It also has input touch, if you use it on Android/iPhone which doesn't react on OnMouseXXX Events

You can remove the Input parts of the controls, if you don't need them.

 using UnityEngine;
 using System.Collections;

 public class MoveObjectScript : MonoBehaviour {
     private Vector3 screenPoint;
     private Vector3 offset;
     private Vector3 curScreenPoint;
     
     void Update() {
         if(Input.touchCount==1) {
             Touch touch = Input.touches[0];
             
             Ray ray = Camera.main.ScreenPointToRay(touch.position);
             RaycastHit rayHit = new RaycastHit();
             bool isOverCrosshair = false;
             if(Physics.Raycast(ray, out rayHit, 20.0f, layer)) {
                 isOverCrosshair = rayHit.transform.name=="Crosshair";
             }

             switch(touch.phase) {
             case TouchPhase.Began:
                 if(isOverCrosshair)
                     OnMouseDown();
                 break;
             case TouchPhase.Canceled:
             case TouchPhase.Ended:
                 if(bMouseDown)
                     OnMouseUp();
                 break;
             case TouchPhase.Moved:
                 OnMouseDrag();
                 break;
             default:
                 break;
             }
         }
     }
     
     void OnMouseDown() {
         Vector3 position = Vector3.zero;
         if(Input.touchCount>0) {
             position = new Vector3(Input.touches[0].position.x, Input.touches[0].position.y, 0);
         } else {
             position = Input.mousePosition;
         }
         // get the start position 
         screenPoint = Camera.main.WorldToScreenPoint(transform.position);
     
         // calculate the offset from the click point to the center of the cross hair
         offset = transform.position - Camera.main.ScreenToWorldPoint(
             new Vector3(position.x, position.y, screenPoint.z));
     }
     
     void OnMouseDrag() {
         // calculate new position
         Vector3 position = Vector3.zero;
         if(Input.touchCount>0) {
             position = new Vector3(Input.touches[0].position.x, Input.touches[0].position.y, 0);
         } else {
             position = Input.mousePosition;
         }

         curScreenPoint = new Vector3(position.x, position.y, screenPoint.z);
         transform.position = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
         // The above one  is for a 2D/2.5D game, where the object will be drag in x and y positon, but be locked on z position.
         // For full 3d movement, use this line instead 
         // transform.position = Camera.main.ScreenToWorldPoint(position) + offset;
     }
 }