Placing a cube on the face of another

place this script inside your scene with a cube and click play :

  void Update () {
 
 
 
         if ( Input.GetMouseButtonDown( 0 ) )
         {
             Ray ray = Camera.main.ScreenPointToRay( Input.mousePosition );
             RaycastHit hit;
             if ( Physics.Raycast( ray , out hit , 2000 ) )
             {
                 // it's better to find the center of the face like this:
                 Vector3 position = hit.transform.position + hit.normal ;
 
                 // calculate the rotation to create the object aligned with the face normal:
                 Quaternion rotation =  Quaternion.FromToRotation( Vector3.up , hit.normal );
                 // create the object at the face center, and perpendicular to it:
                 GameObject Placement = GameObject.CreatePrimitive( PrimitiveType.Cube );
                 Placement.transform.position = position;
                 Placement.transform.rotation = rotation;
 
 
 
                 //Instantiate<( PrimitiveType.Cube as GameObject , position , rotation ) as GameObject;
             }
             else
             {
                 Debug.Log( "nothing" );
             }
         }
 
 
     } 

Not the most descriptive question I've seen... However, if you want some working example, here's a package I wrote some time ago, and I even have a blog post regarding some thoughts about it.

It contains a scene and scripts to generate voxels.

To do what you've asked, "place a block on the face I click on", here's what you will need:

http://unity3d.com/support/documentation/ScriptReference/Input-mousePosition.html

http://unity3d.com/support/documentation/ScriptReference/Camera.ScreenPointToRay.html

Collider.Raycast

(as well as http://unity3d.com/support/documentation/ScriptReference/RaycastHit.html for point and normal)

Object.Instantiate

Good luck!

I made a prefab out of cubes that are allready in unity and this is a code to spawn them in 2D grid. There is also few lines which make middle tile pruple and align camera to the center:

 var tilePrefab : GameObject;
 var y : int = 10; 
 var x : int = 10; 
 var initialColor : Color;
 
 function Start (){
   for (var i : int = 0;i < y; i++) {
     for (var j : int = 0; j < x; j++)
     {
       var tile : GameObject = Instantiate (tilePrefab, Vector3(j, i, 0), Quaternion.identity);
       if (i == y/2 && j == x/2)
       {
         print("test");
         var pos: Vector3 = tile.transform.position; // get the tile position...
         pos.z = Camera.main.transform.position.z; // but keep the camera z 
         Camera.main.transform.position = pos; // move camera to the position
         tile.renderer.material.color = Color.cyan;
 8//,b
       }
     }
   }
 }