OnCollisionStay is called once per frame for every collider/rigidbody that is touching rigidbody/collider.

In contrast to OnTriggerStay, OnCollisionStay is passed the Collision class and not a Collider. The Collision class contains information about contact points, impact velocity etc. If you don't use collisionInfo in the function, leave out the collisionInfo parameter as this avoids unneccessary calculations. Note that collision events are only sent if one of the colliders also has a non-kinematic rigid body attached.

	function OnCollisionStay(collisionInfo : Collision) {
		// Debug-draw all contact points and normals
		for (var contact : ContactPoint in collisionInfo.contacts)
			Debug.DrawRay(contact.point, contact.normal, Color.white);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnCollisionStay(Collision collisionInfo) {
        foreach (ContactPoint contact in collisionInfo.contacts) {
            Debug.DrawRay(contact.point, contact.normal, Color.white);
        }
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnCollisionStay(collisionInfo as Collision) as void:
		for contact as ContactPoint in collisionInfo.contacts:
			Debug.DrawRay(contact.point, contact.normal, Color.white)

OnCollisionStay can be a co-routine, simply use the yield statement in the function.