Add a float material property.

	// Draws 3 meshes with the same material but with different Shininess levels
	var aMesh : Mesh;
	var aMaterial : Material = new Material(Shader.Find("VertexLit"));
	function Update() {
		var materialProperty : MaterialPropertyBlock = new MaterialPropertyBlock();

		// Not shiny
		materialProperty.Clear();
		materialProperty.AddFloat("_Shininess", 1.0);
		Graphics.DrawMesh(aMesh, Vector3(0,0,0), Quaternion.identity,
				  aMaterial, 0, null, 0, materialProperty);
		// More or less shiny
		materialProperty.Clear();
		materialProperty.AddFloat("_Shininess", 0.5);
		Graphics.DrawMesh(aMesh, Vector3(5,0,0), Quaternion.identity,
				  aMaterial, 0, null, 0, materialProperty);
		// Shiny
		materialProperty.Clear();
		materialProperty.AddFloat("_Shininess", 0.0);
		Graphics.DrawMesh(aMesh, Vector3(-5,0,0), Quaternion.identity,
				  aMaterial, 0, null, 0, materialProperty);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Mesh aMesh;
    public Material aMaterial = new Material(Shader.Find("VertexLit"));
    void Update() {
        MaterialPropertyBlock materialProperty = new MaterialPropertyBlock();
        materialProperty.Clear();
        materialProperty.AddFloat("_Shininess", 1.0F);
        Graphics.DrawMesh(aMesh, new Vector3(0, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty);
        materialProperty.Clear();
        materialProperty.AddFloat("_Shininess", 0.5F);
        Graphics.DrawMesh(aMesh, new Vector3(5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty);
        materialProperty.Clear();
        materialProperty.AddFloat("_Shininess", 0.0F);
        Graphics.DrawMesh(aMesh, new Vector3(-5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public aMesh as Mesh

	public aMaterial as Material = Material(Shader.Find('VertexLit'))

	def Update() as void:
		materialProperty as MaterialPropertyBlock = MaterialPropertyBlock()
		materialProperty.Clear()
		materialProperty.AddFloat('_Shininess', 1.0F)
		Graphics.DrawMesh(aMesh, Vector3(0, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty)
		materialProperty.Clear()
		materialProperty.AddFloat('_Shininess', 0.5F)
		Graphics.DrawMesh(aMesh, Vector3(5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty)
		materialProperty.Clear()
		materialProperty.AddFloat('_Shininess', 0.0F)
		Graphics.DrawMesh(aMesh, Vector3(-5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty)

Function variant that takes nameID is faster. If you are adding properties with the same name repeatedly, use Shader.PropertyToID to get unique identifier for the name, and pass the identifier to AddFloat.

	// Draws 3 meshes with the same material but with different Shininess levels
	// Using the material tag ID.
	var aMesh : Mesh;
	var aMaterial : Material = new Material(Shader.Find("VertexLit"));
	function Update() {
		var materialProperty : MaterialPropertyBlock = new MaterialPropertyBlock();
		var tagID : int = Shader.PropertyToID("_Shininess");
		// Not shiny
		materialProperty.Clear();
		materialProperty.AddFloat(tagID, 1.0);
		Graphics.DrawMesh(aMesh, Vector3(0,0,0), Quaternion.identity,
				  aMaterial, 0, null, 0, materialProperty);
		// More or less shiny
		materialProperty.Clear();
		materialProperty.AddFloat(tagID, 0.5);
		Graphics.DrawMesh(aMesh, Vector3(5,0,0), Quaternion.identity,
				  aMaterial, 0, null, 0, materialProperty);
		// Shiny
		materialProperty.Clear();
		materialProperty.AddFloat(tagID, 0.0);
		Graphics.DrawMesh(aMesh, Vector3(-5,0,0), Quaternion.identity,
				  aMaterial, 0, null, 0, materialProperty);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Mesh aMesh;
    public Material aMaterial = new Material(Shader.Find("VertexLit"));
    void Update() {
        MaterialPropertyBlock materialProperty = new MaterialPropertyBlock();
        int tagID = Shader.PropertyToID("_Shininess");
        materialProperty.Clear();
        materialProperty.AddFloat(tagID, 1.0F);
        Graphics.DrawMesh(aMesh, new Vector3(0, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty);
        materialProperty.Clear();
        materialProperty.AddFloat(tagID, 0.5F);
        Graphics.DrawMesh(aMesh, new Vector3(5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty);
        materialProperty.Clear();
        materialProperty.AddFloat(tagID, 0.0F);
        Graphics.DrawMesh(aMesh, new Vector3(-5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public aMesh as Mesh

	public aMaterial as Material = Material(Shader.Find('VertexLit'))

	def Update() as void:
		materialProperty as MaterialPropertyBlock = MaterialPropertyBlock()
		tagID as int = Shader.PropertyToID('_Shininess')
		materialProperty.Clear()
		materialProperty.AddFloat(tagID, 1.0F)
		Graphics.DrawMesh(aMesh, Vector3(0, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty)
		materialProperty.Clear()
		materialProperty.AddFloat(tagID, 0.5F)
		Graphics.DrawMesh(aMesh, Vector3(5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty)
		materialProperty.Clear()
		materialProperty.AddFloat(tagID, 0.0F)
		Graphics.DrawMesh(aMesh, Vector3(-5, 0, 0), Quaternion.identity, aMaterial, 0, null, 0, materialProperty)