This is my full implementation of Click and Drag and Snap to Place. The Snapping bit takes place in the OnMouseUp section, basically, find the center of each slot on the grid, find the location of your Object and subtract from its position to get it to the closest center.

var moving = false;
var posX : float;
var posY : float;
var ray : Ray;

function Update () {

    if(moving == true){
       ray  = Camera.main.ScreenPointToRay (Input.mousePosition);
       transform.position.x = ray.GetPoint(14).x;
     transform.position.y = ray.GetPoint(14).y;
     transform.position.z = 1.1;
    }
}

function OnMouseDown () {
   moving = true;
}

function OnMouseUp () {
    //if out of bounds destroy it
    if(transform.position.x > 1.5 || transform.position.x < -10.5 || transform.position.y > 9.5 || transform.position.y < -.5){
         Destroy(gameObject);
    }
    moving = false;
     //Snap to feature
     posX = transform.position.x;
     posX = Mathf.Round(posX);
    if(posX > transform.position.x){
     posX--;
    }
    posY = transform.position.y;
    posY = Mathf.Round(posY);
    if(posY > transform.position.y){
     posY--;
    }

    transform.position.x = posX +.5;
    transform.position.y = posY +.2;
}

In fact I speak french not very much but I accidentaly found the Download button so heres the direct link to it, tell me if it doesn not work!

LINK

• Felipe