_NavBar.cshtml for MVC4 / Bootstrap 3 project. See http://www.macaw.nl/weblog/2013/5/setting-up-a-solution-with-mvc4-and-twitter-bootstrap

_NavBar.cshtml

@helper ActiveItem(string actionName, string controllerName, string areaName)
{
    if (ViewContext.RouteData.Values["action"].ToString() == actionName &&
            ViewContext.RouteData.Values["controller"].ToString() == controllerName &&
            (ViewContext.RouteData.DataTokens["area"] == null || ViewContext.RouteData.DataTokens["area"].ToString() == areaName))
    {
        @:active
    }
}

<div class="navbar navbar-inverse navbar-fixed-top">
    <div class="container">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">Cloud Auction</a>
        </div>
        <div class="navbar-collapse collapse">
            <ul class="nav navbar-nav">
                <li class="@ActiveItem("Index", "Home", null)">@Html.ActionLink("Home", "Index", "Home")</li>
                <li class="@ActiveItem("About", "Home", null)">@Html.ActionLink("About", "About", "Home")</li>
                <li class="@ActiveItem("Contact", "Home", null)">@Html.ActionLink("Contact", "Contact", "Home")</li>
            </ul>
        </div>
    </div>
</div>