You set the minimap camera to a higher depth than the main camera to get it to render over it.

You set the culling on the minimap cam to depth only so it doesn't render a sky box or anything to cover the rest of the screen.

You then set the viewport normal on the minimap to a corner of the screen.

http://unity3d.com/support/documentation/Components/class-Camera.html

The normalised rect is from 0-1 where 1 is the width of the screen so 0.5 would be half the screen.

You can also make the map extra special by creating a map icons layer and disabling it on the main camera and setting the map cam to only render the terrain layer and map icons layer so you'll see map icons in the minimap and not in the main view.

there are two ways to do this. 1 you can use normalized viewport as described by spinaljack in another answer. 2 you can render the camera to a render texture and use that texture in any 3d plane/GUI you want. this will require you to have unity pro.

Hi there,

Making an minimap is not as simple as it appears when you first think about it. We spent about 3 month implementing a really good working minimap/map system. Its fully customizeable, well documented and super easy to integrate. Maybe this would save you some time?

KGFMapSystem Homepage checkout the screenshots!

KGFMapSystem Assetstore

you make a camera, go into "game" tab (next to "scene") and set the position of the camera (i put it in the upper right corner). next, put it up REALLY high up on top of your player and set the projection to orthographic. now make a c# script that will make the camera follow your player. heres the script i used:

using UnityEngine;

public class CameraFollow : MonoBehaviour {

    public Transform Target;

void LateUpdate()
{
    transform.position = new Vector3(Target.position.x, transform.position.y, Target.position.z);  
}

}

then, set your player as the target. if you see any problems, reply back to me. hope it helps!