I just found this recently. Bátorfi Zsolt posts this XAML code that you can paste into XAMLPad or better yet Robby Ingebretsen's Kaxaml and use to see OpenType features at work. The XAML code can be modified and expanded to change or add fonts or features.
You'll need Windows for this, preferably the 7 beta or Vista, but alternately XP with the .NET 3.0 or 3.5 framework installed.