See the blog post for a description: http://www.caatch.co/invoke-a-method-right-from-a-unity-inspector/

ExposeMethodInEditorAttribute.cs

// The MIT License (MIT) - https://gist.github.com/bcatcho/1926794b7fd6491159e7
// Copyright (c) 2015 Brandon Catcho
using System;

// Place this file in any folder that is or is a descendant of a folder named "Scripts"
namespace CatchCo  
{
   // Restrict to methods only
   [AttributeUsage(AttributeTargets.Method)]
   public class ExposeMethodInEditorAttribute : Attribute
   {
   }
}

MonoBehaviourCustomEditor.cs

// The MIT License (MIT) - https://gist.github.com/bcatcho/1926794b7fd6491159e7
// Copyright (c) 2015 Brandon Catcho
using UnityEngine;  
using UnityEditor;  
using System.Reflection;

// Place this file in any folder that is or is a descendant of a folder named "Editor"
namespace CatchCo  
{
   [CanEditMultipleObjects] // Don't ruin everyone's day
   [CustomEditor(typeof(MonoBehaviour), true)] // Target all MonoBehaviours and descendants
   public class MonoBehaviourCustomEditor : UnityEditor.Editor
   {
      public override void OnInspectorGUI()
      {
         DrawDefaultInspector(); // Draw the normal inspector

         // Currently this will only work in the Play mode. You'll see why
         if (Application.isPlaying)
         {
            // Get the type descriptor for the MonoBehaviour we are drawing
            var type = target.GetType();

            // Iterate over each private or public instance method (no static methods atm)
            foreach (var method in type.GetMethods(BindingFlags.NonPublic|BindingFlags.Public|BindingFlags.Instance))
            {
               // make sure it is decorated by our custom attribute
               var attributes = method.GetCustomAttributes(typeof(ExposeMethodInEditorAttribute), true);
               if (attributes.Length > 0)
               {

                  if (GUILayout.Button("Run: " + method.Name))
                  {
                     // If the user clicks the button, invoke the method immediately.
                     // There are many ways to do this but I chose to use Invoke which only works in Play Mode.
                     ((MonoBehaviour)target).Invoke(method.Name, 0f);
                  }
               }
            }
         }
      }
   }
}