Put CSS class depending on selected controller

I generally create an action link html helper for accomplishing this task. Note, that I mark the link itself as "selected" vs the list item.

public static class ActionLinkHelpers
{
    public static MvcHtmlString SelectedActionLink(this HtmlHelper helper, string linkText, string actionName, string controllerName)
    {
        var controller = (string) helper.ViewContext.RouteData.Values["controller"];
        if (string.Compare(controller, controllerName, StringComparison.InvariantCultureIgnoreCase) == 0)
        {
            return helper.ActionLink(linkText, actionName, controllerName, null, new { Class = "selected" }); 
        }

        return helper.ActionLink(linkText, actionName, controllerName);
    }
}

After you have an action link helper setup within your project your list would look as follows:

<li>@Html.SelectedActionLink("Home", "index", "Home")</li>
<li>@Html.SelectedActionLink("Cars", "index", "Car")</li>

EDIT:

In order to use a custom helper MVC must be aware of it. Add a new folder to your project "HtmlHelpers" for example and place this class inside of it. From there you need to add a line to the /Views/Web.config:

<pages pageBaseType="System.Web.Mvc.WebViewPage">
  <namespaces>
    <add namespace="YourNameSpace.HtmlHelpers"/>
  </namespaces>
</pages>