Gizmos.DrawLine

Description

Draws a line starting at from towards to.

    var target : Transform;
    function OnDrawGizmosSelected () {
        if(target != null) {
            // Draws a blue line from this transform to the target
            Gizmos.color = Color.blue;
            Gizmos.DrawLine (transform.position, target.position);
        }
    }

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform target;
    void OnDrawGizmosSelected() {
        if (target != null) {
            Gizmos.color = Color.blue;
            Gizmos.DrawLine(transform.position, target.position);
        }
    }
}