Magnet.cs

   ///////////////////////////////////////////
  // "Magnet.cs" - Runtime object snapping //
 // Written by Michael Wion on 01/15/2015 //
///////////////////////////////////////////

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

[System.Serializable]
public struct MagnetizedItem
{
    public Vector3 polarity;
    public Vector3 offset;
    public GameObject[] allowed;

    public void Attach (GameObject obj) {
        Magnet m = Magnet.GetInstance ();
        Transform mTransform = m.transform;
        GameObject mObject = mTransform.gameObject;
        Bounds mBounds = mObject.renderer.bounds;
        Bounds oBounds = obj.renderer.bounds;
        polarity.Normalize();
        Vector3 direction = new Vector3(
            ((mBounds.size.x / 2) + (oBounds.size.x / 2)) * polarity.x,
            ((mBounds.size.y / 2) + (oBounds.size.y / 2)) * polarity.y,
            ((mBounds.size.z / 2) + (oBounds.size.z / 2)) * polarity.z
        );
        obj.transform.position = (mTransform.position + direction) + offset;
    }
}

public class Magnet : MonoBehaviour
{
    private static Magnet _instance;
    public List<MagnetizedItem> items;

    public static Magnet GetInstance() {
        return _instance;
    }

    public void Awake() {
        _instance = this;
        if(items == null) items = new List<MagnetizedItem>();
    }

    void Start() {
        foreach(MagnetizedItem item in items) {
            foreach(GameObject obj in item.allowed) {
                item.Attach(obj);
            }
        }
    }
}