主要控制相机在场景里自由的平移，旋转，缩放，而不跟踪一个指定GameObject的实现。
新建场景，将此脚本拖放到一个摄像机上即可，鼠标中键缩放，中键按下移动，右键旋转。
代码如下：

using UnityEngine;
using System.Collections;

namespace Gavin
{
    public class FreeCamera : MonoBehaviour {
       //旋转变量;
        private float m_deltX = 0f;
        private float m_deltY = 0f;
        //缩放变量;
        private float m_distance = 10f;
        private float m_mSpeed = 5f;
        //移动变量;
        private Vector3 m_mouseMovePos = Vector3.zero;
      
        void Start()
        {
            camera.transform.localPosition = new Vector3(0, m_distance, 0);
        }
   
        void Update () {
            //鼠标右键点下控制相机旋转;
            if (Input.GetMouseButton(1))
            {
                m_deltX += Input.GetAxis("Mouse X") * m_mSpeed;
                m_deltY -= Input.GetAxis("Mouse Y") * m_mSpeed;
                m_deltX = ClampAngle(m_deltX, -360, 360);
                m_deltY = ClampAngle(m_deltY, -70, 70);
                camera.transform.rotation = Quaternion.Euler(m_deltY, m_deltX, 0);
            }

            //鼠标中键点下场景缩放;
            if (Input.GetAxis("Mouse ScrollWheel") != 0)
            {
                //自由缩放方式;
                m_distance = Input.GetAxis("Mouse ScrollWheel") * 10f;
                camera.transform.localPosition = camera.transform.position + camera.transform.forward * m_distance;
            }
            //鼠标点击场景移动;
            if (Input.GetMouseButtonDown(2))
            {
                Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);//从摄像机发出到点击坐标的射线
                RaycastHit hitInfo;
                if (Physics.Raycast(ray, out hitInfo))
                {
                    m_mouseMovePos = hitInfo.point;
                }
            }else if (Input.GetMouseButton(2))
            {
                Vector3 p = Vector3.zero;
                Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);//从摄像机发出到点击坐标的射线
                RaycastHit hitInfo;
                if (Physics.Raycast(ray, out hitInfo))
                {
                    p = hitInfo.point - m_mouseMovePos;
                    p.y = 0f;
                }
                camera.transform.localPosition = camera.transform.position - p * 0.05f; //在原有的位置上，加上偏移的位置量;
            }

            //相机复位远点;
            if (Input.GetKey(KeyCode.Space))
            {
                m_distance = 10.0f;
                camera.transform.localPosition = new Vector3(0, m_distance, 0);
            }
        }

        //规划角度;
        float ClampAngle(float angle, float minAngle, float maxAgnle)
        {
            if (angle <= -360)
                angle += 360;
            if (angle >= 360)
                angle -= 360;

            return Mathf.Clamp(angle, minAngle, maxAgnle);
        }
    }
}