Is it possible to display a view from another controller?
Say for example I have a CategoriesController and a Category/NotFound.aspx view.
Yes. By default, ASP.NET MVC checks first in \Views\[Controller_Dir]\, but after that, if it doesn't find the view, it checks in \Views\Shared.
The shared directory is there specifically to share Views across multiple controllers. Just add your View to the Shared subdirectory and you're good to go.
If you do return View("~/Views/Wherever/SomeDir/MyView.aspx") You can return any View you'd like.