If you want to retrieve the currently
authenticated user from the SharePoint context, you need to remain
within the SharePoint context. This means hosting your custom web
application within SharePoint (see http://msdn.microsoft.com/en-us/library/cc297200.aspx).
Then from your custom application reference Microsoft.SharePoint and
use the SPContext object to retrieve the user name. For example:
You can still use the Page Viewer Web Part to reference the URL of the site, now located within the SharePoint context.