NEW: OS X 10.9 Mavericks compatible
Windat Opener is also available for the iPhone and iPad; for more information, see here.
WinDat Opener is the easiest way to deal with those pesky winmail.dat files you keep on getting as attachments sent from Windows machines. But WinDat Opener is also powerful:
Microsoft e-mail programs written for Windows, such as Outlook, sometimes send e-mails in the Transport Neutral Encapsulation Format (TNEF). Most other e-mail programs, including Apple Mail and Microsoft Entourage, do not understand TNEF. If your e-mail program doesn't understand TNEF, instead of seeing the e-mail and/or attachment, you may only see an attachment named "winmail.dat" or "Part 1.2" that you cannot open. Also, sometimes you may receive a TNEF attachment with a generic name such as ATT00008.dat or ATT00005.eml instead. While almost all attachments named "winmail.dat" are TNEF, you could receive non-TNEF attachments with names ending ".dat" or ".eml", or which are named or labeled "Part 1.2".
Looking for a iPhone and iPad version of WinDat Opener? WinDat Opener lives here: WinDat Opener for iOS
OS X 10.6.6 (Snow Leopard) or later