“tnefDD is a program for Mac OS X that extracts attachments from TNEF format files. It is built as a modern OS X application with all the ease of use features like drag-and-drop that OS X user expect, and runs on Intel- and PowerPC-based Mac computers.”

via tnefDD.

This is a great tool for extracting TNEF/winmail.dat-files that Outlook tends to pack everything inside.