This is probably my blog post with the most acronyms in it. In any case, on to the contents.
If a user navigation to a .xaml file using Internet Explorer 7 and .NET Framework 3.0 is not installed, then it will offer the user to install it. This is very handy if you, like me, are too lazy to look up the installation point (it's here, by the way).
This snippet can be saved into a file and double-clicked to check whether the machine has the required bits or prompt for installation. In fact, if you are as lazy as I am, you will likely copy this, open a prompt, and go copy con xaml.xaml, Alt+Space (to bring up the window menu), E (for Edit), P (for Paste), Enter, Ctrl+Z, then start xaml.xaml.
FontFamily='Cambria'><Bold>Windows Presentation Foundation</Bold> is installed!</TextBlock>
Update 2007-01-27: adding the XAML as an attachment for additional convenience.