Ninject is very cool and lightweight DI container.
But you can not always inject objects through constructor – sometimes you must have parameter-less constructor, like in MvcApplication object.
In that case you can use DependencyResolver from System.Web.Mvc namespace from System.Web assembly

_userDataService = 
DependencyResolver.Current.GetService<IUserDataService>();

Unfortunately MSDN documentation does not have much, only link to this blog post: http://bradwilson.typepad.com/blog/2010/10/service-location-pt5-idependencyresolver.html