For the urls like www.website.com/user/username, the following route can be used.

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
            "UserProfile",
            "Users/{username}",
            new { controller = "User", action = "Index", username = string.Empty }
        );

        routes.MapRoute(
            "Default",
            "{controller}/{action}/{id}",
            new { controller = "Home", action = "Index", id = UrlParameter.Optional }
        );
    }
}
public class UserController : Controller
{
    public ActionResult Index(string username)
    {
        return View();
    }
}

Further Reading :
User routing in ASP.NET MVC for urls like www.website.com/users/jeffAtwood