Have you ever recieved an email with a winmail.dat attachment from your contact? Ever wondered why that came?

From Microsoft KB Articles 278061 and 138053,

Winmail.dat file is used to preserve formatting that the sending client includes in the message, but the receiving client does not recognize the Winmail.dat file. In Outlook, the Winmail.dat file includes Rich Text Formatting (RTF) instructions. This type of formatting is used with the Microsoft Outlook Rich Text format and when you use Microsoft Word as your e-mail editor.
The Winmail.dat file contains Exchange Server RTF information for the message, and may appear to the recipient as a binary file. It is not useful to non-Exchange Server recipients.
This usually happens when sending emails from a MAPI client like Outlook to a Webmail service like Hotmail, Yahoo, etc.

So if you happen to recieve such as attachment from someone, you can open it with the help of a freeware application called Winmail Opener. With Winmail Opener you can view the rich text message contents and attachments embedded into this file.

Features of this application include:

-Supports RTF message text.
-Drag-and-drop TNEF-encoded files into Winmail Opener for opening them.
-Drag-and-drop decoded attachments from Winmail Opener to any location for saving them.
-Supports command line mode for batch decoding.

Download here