DraggableRigidbodyController.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class DraggableRigidbodyController : MonoBehaviour
{
    public List<string> draggableTags;
    [Range(0.5f,0.99f)]
    public float smoothness = 0.9f;
    public float lift = 2f;
    Rigidbody curObj = null;
    public bool makeKinematic = true;
    Plane movePlane;

    void TryClick ()
    {
        Ray camRay = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if (Physics.Raycast(camRay,out hit))
        {
            if (hit.rigidbody == null)
                return;

            if (draggableTags.Contains(hit.rigidbody.tag))
            {
                curObj = hit.rigidbody;
                if (makeKinematic)
                    curObj.isKinematic = true;
                movePlane = new Plane(Vector3.up,curObj.transform.position);
            }
        }
    }

    void FreeObject ()
    {
        if (makeKinematic)
            curObj.isKinematic = false;

        curObj = null;
    }

    void Update ()
    {
        if (Input.GetMouseButtonDown(0) && curObj == null)
        {
            TryClick();
        }

        if (Input.GetMouseButtonUp(0) && curObj != null)
        {
            FreeObject ();
        }

        if (curObj == null)
            return;

        Ray camRay = Camera.main.ScreenPointToRay(Input.mousePosition);
        float hitDist;
        if (movePlane.Raycast(camRay,out hitDist))
        {
            Vector3 newPos = camRay.GetPoint(hitDist)+Vector3.up*lift;
            curObj.MovePosition(Vector3.Lerp(curObj.transform.position, newPos,1f - smoothness));
        }
    }
}