DraggableRigidbody.cs

using UnityEngine;
using System.Collections;

public class DraggableRigidbody : MonoBehaviour
{
    bool dragging = false;
    public bool makeKinematic = true;
    Plane movePlane;

    void OnMouseDown ()
    {
        if (makeKinematic)
            rigidbody.isKinematic = true;
        dragging = true;
        movePlane = new Plane(-Camera.main.transform.forward,transform.position);
    }

    void Update ()
    {
        if (!dragging || !this.enabled)
            return;

        if (Input.GetMouseButtonUp(0))
        {
            dragging = false;
            if (makeKinematic)
                rigidbody.isKinematic = false;
        }

        Ray camRay = Camera.main.ScreenPointToRay(Input.mousePosition);
        float hitDist;
        if (movePlane.Raycast(camRay,out hitDist))
        {
            Vector3 newPos = camRay.GetPoint(hitDist);
            rigidbody.MovePosition(Vector3.Lerp(transform.position, newPos,0.5f));
        }
    }
}