Instantiates prefab when any rigid body enters the trigger

// Instantiates prefab when any rigid body enters the trigger.
// It preserves the prefab's original position and rotation.

var prefab : Transform;

function OnTriggerEnter () {
Instantiate (prefab);
}

Instantiate a projectile for a Shooter Game

// Instantiate a rigidbody then set the velocity

var projectile : Rigidbody;

function Update () {
// Ctrl was pressed, launch a projectile
if (Input.GetButtonDown("Fire1")) {
// Instantiate the projectile at the position and rotation of this transform
var clone : Rigidbody;
clone = Instantiate(projectile, transform.position, transform.rotation);

// Give the cloned object an initial velocity along the current
// object's Z axis
clone.velocity = transform.TransformDirection (Vector3.forward * 10);
}
}

Particles Instantiate Idea

Let's try to instantiate some particle effects on Trigger
Like fireworks and other nice effects

// Instantiates 10 copies of prefab each 2 units apart from each other

Object.Instantiate  

static function Instantiate (original : Object, position : Vector3, rotation : Quaternion) : Object

Description

Clones the object original and returns the clone.

Clones the object original, places it at position and sets the rotation to rotation, then returns the cloned object. This is essentially the same as using duplicate command (cmd-d) in Unity and then moving the object to the given location. If a game object, component or script instance is passed, Instantiate will clone the entire game object hierarchy, with all children cloned as well. All game objects are activated.
See Also: In depth Prefab Instantiate discussion

// Instantiates 10 copies of prefab each 2 units apart from each other

var prefab : Transform;

for (var i : int = 0;i < 10; i++) {
Instantiate (prefab, Vector3(i * 2.0, 0, 0), Quaternion.identity);
}

Instantiate is most commonly used to instantiate projectiles, AI Enemies, particle explosions or wrecked object replacements.