• Winmail.dat Reader → http://www.kopf.com.br/winmail/winmail.php

Winmail.dat files are in fact TNEF attachments (transport neutral
encapsulation format) sent by the Microst Outlook, Outlook Express
or Microsoft Exchange applications.

• http://sourceforge.net/projects/tnef/

TNEF provides a way to unpack those pesky Microsoft MS-TNEF MIME
attachments. It operates like tar in order to upack any files which
may have been put into the MS-TNEF attachment instead of being
attached seperately.

• Opening winmail.dat files →
  http://www.linuxworld.com/columnists/2004/112904drinternet.html