DragRigidbody

using UnityEngine;
using System.Collections;

public class DragRigidbody : MonoBehaviour
{
    public float maxDistance = 100.0f;

    public float spring = 50.0f;
    public float damper = 5.0f;
    public float drag = 10.0f;
    public float angularDrag = 5.0f;
    public float distance = 0.2f;
    public bool attachToCenterOfMass = false;

    private SpringJoint springJoint;

    void Update()
    {
        if(!Input.GetMouseButtonDown(0))
            return;

        Camera mainCamera = FindCamera();

        RaycastHit hit;
        if(!Physics.Raycast(mainCamera.ScreenPointToRay(Input.mousePosition), out hit, maxDistance))
            return;
        if(!hit.rigidbody || hit.rigidbody.isKinematic)
            return;

        if(!springJoint)
        {
            GameObject go = new GameObject("Rigidbody dragger");
            Rigidbody body = go.AddComponent<Rigidbody>();
            body.isKinematic = true;
            springJoint = go.AddComponent<SpringJoint>();
        }

        springJoint.transform.position = hit.point;
        if(attachToCenterOfMass)
        {
            Vector3 anchor = transform.TransformDirection(hit.rigidbody.centerOfMass) + hit.rigidbody.transform.position;
            anchor = springJoint.transform.InverseTransformPoint(anchor);
            springJoint.anchor = anchor;
        }
        else
        {
            springJoint.anchor = Vector3.zero;
        }

        springJoint.spring = spring;
        springJoint.damper = damper;
        springJoint.maxDistance = distance;
        springJoint.connectedBody = hit.rigidbody;

        StartCoroutine(DragObject(hit.distance));
    }

    IEnumerator DragObject(float distance)
    {
        float oldDrag             = springJoint.connectedBody.drag;
        float oldAngularDrag     = springJoint.connectedBody.angularDrag;
        springJoint.connectedBody.drag             = this.drag;
        springJoint.connectedBody.angularDrag     = this.angularDrag;
        Camera cam = FindCamera();

        while(Input.GetMouseButton(0))
        {
            Ray ray = cam.ScreenPointToRay(Input.mousePosition);
            springJoint.transform.position = ray.GetPoint(distance);
            yield return null;
        }

        if(springJoint.connectedBody)
        {
            springJoint.connectedBody.drag             = oldDrag;
            springJoint.connectedBody.angularDrag     = oldAngularDrag;
            springJoint.connectedBody                 = null;
        }
    }

    Camera FindCamera()
    {
        if (camera)
            return camera;
        else
            return Camera.main;
    }
}