public ActionResult Index()
{
    ViewData["foo"] = "bar";
    return View();
}

<div>@ViewData["foo"]</div>

public ActionResult Index()
{
    ViewBag.foo = "bar";
    return View();
}

<div>@ViewBag.foo</div>

public class MyViewModel
{
    public string Foo { get; set; }
}

public ActionResult Index()
{
    var model = new MyViewModel { Foo = "bar" };
    return View(model);
}

@model MyViewModel
<div>@Html.DisplayFor(x => x.Foo)</div>

public ActionResult Foo()
{
    TempData["foo"] = "bar";
    return RedirectToAction("bar");
}

public ActionResult Bar()
{
    var value = TempData["foo"] as string;
    // use the value here. If you need to pass it to the view you could
    // use ViewData/ViewBag (I can't believe I said that but I will leave it for the moment)
    return View();
}