Xtras are supported and available on the following products:
However, both Authorware and Freehand have been left in limbo by current owner Adobe, and Director always had the broadest support for Xtras (all of the three types of Xtras could be used).
Xtras use the Macromedia Open Architecture http://www.adobe.com/support/xtras/info/moa.html which was designed to allow easy creation of interchangeable components between Macromedia products.
Xtras for Win32 must have an .X32 file extension. Xtras for the Macintosh generally have an .XTR extension. The file extension *.X16 is reserved for Xtras for Win16.
Adobe's main Xtras page - http://www.adobe.com/products/xtras/ Freehand Xtras Development Kit - http://www.adobe.com/support/xtras/xdks/fhxdk.html
You can learn which Xtras are available in your copy of Director by typing ShowXlib in the message window.