Quaternion.Slerp  

// Interpolates rotation between the rotations "from" and "to"
// (Choose from and to not to be the same as 
// the object you attach this script to)

var from : Transform;
var to : Transform;
var speed = 0.1;
function Update () {
    transform.rotation =
      Quaternion.Slerp (from.rotation, to.rotation, Time.time * speed);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Transform from;
    public Transform to;
    public float speed = 0.1F;
    void Update() {
        transform.rotation = Quaternion.Slerp(from.rotation, to.rotation, Time.time * speed);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public from as Transform

    public to as Transform

    public speed as single = 0.1F

    def Update():
        transform.rotation = Quaternion.Slerp(from.rotation, to.rotation, (Time.time * speed))