public class MvcApplication : System.Web.HttpApplication
{
    private static readonly ISessionFactory sessionFactory = BuildSessionFactory();

    public static ISession CurrentSession
    {
        get{ return HttpContext.Current.Items["NHibernateSession"] as ISession;}
        set { HttpContext.Current.Items["NHibernateSession"] = value; }
    }

    public MvcApplication()
    {
        BeginRequest += (sender, args) =>
        {
            CurrentSession = sessionFactory.OpenSession();
        };
        EndRequest += (o, eventArgs) =>
        {
            var session = CurrentSession;
            if (session != null)
            {
                session.Dispose();
            }
        };
    }

    protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();

        RegisterGlobalFilters(GlobalFilters.Filters);
        RegisterRoutes(RouteTable.Routes);
    }

    private static ISessionFactory BuildSessionFactory()
    {
        return new Configuration()
            .Configure()
            .BuildSessionFactory();
    }
}

And in the controller, we have something like this:

public class HomeController : SessionController
{
    public ActionResult Blog(int id)
    {
        var blog = MvcApplication.CurrentSession.Get<Blog>(id);

        return Json(blog, JsonRequestBehavior.AllowGet);
    }
}

This code is valid, it works, it follows best practices and I actually recommend using something very similar here.

It also annoyed me when I wrote it now, enough to write a series of blog posts detailing how to fix this.