ContactPoint.otherCollider  

// Prints the name of the collider this transform collided with
function OnCollisionEnter(other : Collision) { 
    print("This collider collided with: " + other.contacts[0].otherCollider.name);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnCollisionEnter(Collision other) {
        print("This collider collided with: " + other.contacts[0].otherCollider.name);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnCollisionEnter(other as Collision):
        print(('This collider collided with: ' + other.contacts[0].otherCollider.name))