ItemHolder.cs

using UnityEngine;
using System.Collections;

/// <summary>
/// Create a new scene, drag this script to any game object, then Play.
/// </summary>
public class ItemHolder : MonoBehaviour
{
    #region Private members
    /// <summary>
    /// The surface which an item is moved on (while dragging mouse).
    /// </summary>
    GameObject m_Surface;

    /// <summary>
    /// The item which is being held by mouse.
    /// </summary>
    Transform m_Item;

    /// <summary>
    /// The offset from center of an item to the click-point. 
    /// </summary>
    Vector3 m_Offset;

    /// <summary>
    /// The layer mask of surface (using for Physics.Raycast).
    /// </summary>
    int m_SurfaceLayerMask;

    /// <summary>
    /// The layer mask of items (using for Physics.Raycast).
    /// </summary>
    int m_ItemLayerMask;
    #endregion

    #region Unity cycle
    void Start()
    {
        CreateItems();
        CreateSurface();
    }

    void Update()
    {
        CheckMouseDown();
        CheckMouseUp();
        CheckMouseDrag();
    }
    #endregion

    #region Start methods
    void CreateItems()
    {
        for (int i = 0; i < 10; i++)
        {
            CreateItem(i);
        }
    }

    void CreateSurface()
    {
        m_Surface = GameObject.CreatePrimitive(PrimitiveType.Quad);
        m_Surface.name = "[SURFACE]";
        m_Surface.layer = 1;
        m_Surface.transform.localScale = Vector3.one * 30;
        m_Surface.GetComponent<Renderer>().enabled = false;

        m_SurfaceLayerMask = 1 << m_Surface.layer;
    }
    #endregion

    #region CreateItem methods
    void CreateItem(int index)
    {
        GameObject item = GameObject.CreatePrimitive(GetShape(index));
        item.layer = 0;
        item.transform.position = RandomPosition();

        m_ItemLayerMask = 1 << item.layer;
    }

    PrimitiveType GetShape(int index)
    {
        int shapeIndex = index % 3;

        switch (shapeIndex)
        {
            case 0:
                return PrimitiveType.Sphere;
            case 1:
                return PrimitiveType.Cube;
            case 2:
                return PrimitiveType.Cylinder;
        }

        return PrimitiveType.Cube;
    }

    Vector3 RandomPosition()
    {
        return new Vector3(Random.Range(-2f, 2f), Random.Range(-2f, 2f), Random.Range(-2f, 2f));
    }
    #endregion

    #region Update methods
    void CheckMouseDown()
    {
        if (Input.GetMouseButtonDown(0))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            RaycastHit hit;

            // Raycast to items only to check which item will be held (when mouse down).
            if (Physics.Raycast(ray, out hit, 100, m_ItemLayerMask))
            {
                m_Item = hit.collider.gameObject.transform;
                m_Offset = hit.point - m_Item.position;
                m_Surface.transform.position = hit.point;
            }
        }
    }

    void CheckMouseUp()
    {
        if (Input.GetMouseButtonUp(0))
        {
            m_Item = null;
        }
    }

    void CheckMouseDrag()
    {
        if (Input.GetMouseButton(0))
        {
            if (m_Item != null)
            {
                Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
                RaycastHit hit;

                // Raycast to the surface only to determine new position of the item (while dragging mouse).
                if (Physics.Raycast(ray, out hit, 100, m_SurfaceLayerMask))
                {
                    m_Item.position = hit.point - m_Offset;
                }
            }
        }
    }
    #endregion
}