Useful for getting world cordinates of screen limits in both ortho and perspective camera types.

ScreenManager.cs

using UnityEngine;
using System.Collections;

public class ScreenManager : MonoBehaviour
{
	// mouse position in screen cordinates.
	public Vector3 screenPosition;
	// mouse position in world space.
	public Vector3 worldPosition;
	// Bounds of the screen in world space.
	public Vector2 screenBounds;

	// Use this for initialization
	void Awake ()
	{
		if(Camera.main.isOrthoGraphic)
		{
			screenBounds = Camera.main.ScreenToWorldPoint(new Vector3(Screen.width,Screen.height));
		}else{
			screenBounds = GetWorldPositionOnPlane(new Vector3(Screen.width,Screen.height,0));
		}
	}

	// Update is called once per frame
	void Update ()
	{
		// Mouse input in screen cordinates
		screenPosition.x = Input.mousePosition.x;
		screenPosition.z = 0f;
		screenPosition.y = (Screen.height - Input.mousePosition.y);

		// Mouse input in world cordinates based on camera perspective
		if(Camera.main.isOrthoGraphic)
		{
			worldPosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
			worldPosition.z = 0f;
		}else{
			worldPosition = GetWorldPositionOnPlane(Input.mousePosition);
		}
	}

	// Gets world cordinates from screen cordinates
	public Vector3 GetWorldPositionOnPlane(Vector3 screenPosition) {
		Ray ray = Camera.main.ScreenPointToRay(screenPosition);
		Plane xy = new Plane(Vector3.forward, new Vector3(0, 0, 0));
		float distance;
		xy.Raycast(ray, out distance);
		return ray.GetPoint(distance);
	}
}