xFx Viewer is an evolution of xAP Message Viewer. The original xAP Message Viewer was developed by Stuart Booth as part of his xAP Framework initiative. Development of the viewer has now been taken up by Edward Pearson who has ported the xAP Framework to .Net 2.0, upgraded Viewer to take advantage of the new library and incorporated a number of enhancements to enable Viewer to perform in the complex xAP environments that the xAP community is now creating.
Follow this link for the background of the development of the earlier versions of xAP Message Viewer.
The latest xFx Viewer is available for download from Edward's site.