If you are writing Windows Store Apps (as they are called now), and you’ve written Windows Phone/Silverlight/WPF apps before, you may be missing System.Windows.Interactivity which allows developers to attach behaviors and triggers (among other things) to UI elements in their pages. This DLL ususally comes with Expression blend, but is missing on Windows 8.
I did a quick-and-dirty port of that DLL to WinRT to allow Behaviors and Triggers to work.
You are welcome to download it.
Important note: The namespace for the DLL is NotSystem.Windows.Interactivity – for some reason it was impossible for me to get the designers to work when the namespace was System.Windows.Interactivity.