The barycentric coordinate of the triangle we hit.

This lets you interpolate any of the vertex data along the 3 axes.

	// Attach this script to a camera and it will 
	// draw a debug line pointing outward from the normal 
	function Update () { 
	   // Only if we hit something, do we continue 
	   var hit : RaycastHit; 
	   if (!Physics.Raycast (Camera.main.ScreenPointToRay(Input.mousePosition), hit)) 
	      return; 
	   // Just in case, also make sure the collider also has a renderer 
	   // material and texture 
	   var meshCollider = hit.collider as MeshCollider; 
	   if (meshCollider == null || meshCollider.sharedMesh == null) 
	      return; 
	   var mesh : Mesh = meshCollider.sharedMesh; 
	   var normals = mesh.normals; 
	   var triangles = mesh.triangles; 
	   // Extract local space normals of the triangle we hit 
	   var n0 = normals[triangles[hit.triangleIndex * 3 + 0]]; 
	   var n1 = normals[triangles[hit.triangleIndex * 3 + 1]];    
	   var n2 = normals[triangles[hit.triangleIndex * 3 + 2]];    
	    
	   // interpolate using the barycentric coordinate of the hitpoint 
	   var baryCenter = hit.barycentricCoordinate; 
	   // Use barycentric coordinate to interpolate normal 
	   var interpolatedNormal = n0 * baryCenter.x + n1 * baryCenter.y + n2 * baryCenter.z; 
	   // normalize the interpolated normal 
	   interpolatedNormal =  interpolatedNormal.normalized; 
	    
	   // Transform local space normals to world space 
	   var hitTransform : Transform = hit.collider.transform; 
	   interpolatedNormal = hitTransform.TransformDirection(interpolatedNormal); 
	   // Display with Debug.DrawLine 
	   Debug.DrawRay(hit.point, interpolatedNormal); 
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        RaycastHit hit;
        if (!Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hit))
            return;
        
        MeshCollider meshCollider = hit.collider as MeshCollider;
        if (meshCollider == null || meshCollider.sharedMesh == null)
            return;
        
        Mesh mesh = meshCollider.sharedMesh;
        Vector3[] normals = mesh.normals;
        int[] triangles = mesh.triangles;
        Vector3 n0 = normals[triangles[hit.triangleIndex * 3 + 0]];
        Vector3 n1 = normals[triangles[hit.triangleIndex * 3 + 1]];
        Vector3 n2 = normals[triangles[hit.triangleIndex * 3 + 2]];
        Vector3 baryCenter = hit.barycentricCoordinate;
        Vector3 interpolatedNormal = n0 * baryCenter.x + n1 * baryCenter.y + n2 * baryCenter.z;
        interpolatedNormal = interpolatedNormal.normalized;
        Transform hitTransform = hit.collider.transform;
        interpolatedNormal = hitTransform.TransformDirection(interpolatedNormal);
        Debug.DrawRay(hit.point, interpolatedNormal);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		hit as RaycastHit
		if not Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), ):
			return
		meshCollider as MeshCollider = (hit.collider as MeshCollider)
		if (meshCollider == null) or (meshCollider.sharedMesh == null):
			return
		mesh as Mesh = meshCollider.sharedMesh
		normals as (Vector3) = mesh.normals
		triangles as (int) = mesh.triangles
		n0 as Vector3 = normals[triangles[((hit.triangleIndex * 3) + 0)]]
		n1 as Vector3 = normals[triangles[((hit.triangleIndex * 3) + 1)]]
		n2 as Vector3 = normals[triangles[((hit.triangleIndex * 3) + 2)]]
		baryCenter as Vector3 = hit.barycentricCoordinate
		interpolatedNormal as Vector3 = (((n0 * baryCenter.x) + (n1 * baryCenter.y)) + (n2 * baryCenter.z))
		interpolatedNormal = interpolatedNormal.normalized
		hitTransform as Transform = hit.collider.transform
		interpolatedNormal = hitTransform.TransformDirection(interpolatedNormal)
		Debug.DrawRay(hit.point, interpolatedNormal)