ScreenPointToRay(position: Vector3): Ray;
Ray ScreenPointToRay(Vector3 position);
def ScreenPointToRay(position as Vector3) as Ray

Returns a ray going from camera through a screen point.

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

Screenspace is defined in pixels. The bottom-left of the screen is (0,0); the right-top is (pixelWidth,pixelHeight).
	// Draws a line in the scene view going through a point 200 pixels
	// from the lower-left corner of the screen
	function Update () {
		var ray : Ray = camera.ScreenPointToRay (Vector3(200,200,0));
		Debug.DrawRay (ray.origin, ray.direction * 10, Color.yellow);
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Ray ray = camera.ScreenPointToRay(new Vector3(200, 200, 0));
        Debug.DrawRay(ray.origin, ray.direction * 10, Color.yellow);
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		ray as Ray = camera.ScreenPointToRay(Vector3(200, 200, 0))
		Debug.DrawRay(ray.origin, (ray.direction * 10), Color.yellow)