ParallaxScroll.cs

using UnityEngine;
using System.Collections;

public class ParallaxScroll : MonoBehaviour {
  
  public float furthestDepth = 100.0F;
  public float depthStep = 10.0F;
  public float baseSpeed = 0.001F;
  public int baseSpeedup = 2;

  public GameObject[] parallaxLayerPrefabs = new GameObject[2];
  private GameObject[] parallaxLayers      = new GameObject[2];
  
  private Transform lastCameraPosition;
  
  void Awake() {
    for (int i = 0; i < parallaxLayerPrefabs.Length; i++) {
      parallaxLayers[i] = Instantiate(parallaxLayerPrefabs[i], new Vector3(0.0F, 0.0F, furthestDepth), Quaternion.identity) as GameObject;
      furthestDepth -= depthStep;
    }
    lastCameraPosition = Camera.main.transform;
  }
  
  void Update() {
    float distance = Vector3.Distance(transform.position, lastCameraPosition.position);
    
    if (distance == 0.0F) return;

    float speed = baseSpeed;
    for (int i = 0; i < parallaxLayers.Length; i++) {
      parallaxLayers[i].transform.position = new Vector3(lastCameraPosition.position.x * (distance * speed), lastCameraPosition.position.y * (distance * speed), parallaxLayers[i].transform.position.z);
      speed *= baseSpeedup; // speed increases the closer to the camera the layer gets
    }
    lastCameraPosition = Camera.main.transform;
  }

}