Novel Windows XP thing: When you try to open a file of an unknown type, Win XP will let you pick an application to open it with, as always, or give you the option of learning more about what type of file it may be on their website. Those of you who know a bit of my personality know that I have a weakness for anything that leverages networked knowledge. And this is kind of a neat solution to the file-extension kludge that Mac users find so detestable about Windows. (Cruel fate that OS X uses extensions now, but not entirely unexpected.)
To see a sample message for a totally unknown type, you can see what ".anil" files return, or for one that's a little more reasonable, you can see the message that a user would presumably get for PDF files if they didn't have Adobe Acrobat installed.