Camera.ViewportPointToRay

function ViewportPointToRay (position : Vector3) : Ray

Description

Returns a ray going from camera through a viewport point.

Resulting ray is in world space, starting on the near plane of the camera and going through position's (x,y) coordinates on the viewport (position.z is ignored).

Viewport coordinates are normalized and relative to the camera. The bottom-left of the camera is (0,0); the top-right is (1,1).

JavaScript
    // Prints the name of the object camera is directly looking at
function Update () {
// Get the ray going through the center of the screen
var ray : Ray = camera.ViewportPointToRay (Vector3(0.5,0.5,0));
// Do a raycast
var hit : RaycastHit;
if (Physics.Raycast (ray, hit))
print ("I'm looking at " + hit.transform.name);
else
print ("I'm looking at nothing!");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Update() {
Ray ray = camera.ViewportPointToRay(new Vector3(0.5F, 0.5F, 0));
RaycastHit hit;
if (Physics.Raycast(ray, out hit))
print("I'm looking at " + hit.transform.name);
else
print("I'm looking at nothing!");
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Update():
ray as Ray = camera.ViewportPointToRay(Vector3(0.5F, 0.5F, 0))
hit as RaycastHit
if Physics.Raycast(ray, hit):
print(('I\'m looking at ' + hit.transform.name))
else:
print('I\'m looking at nothing!')