From ProcessText Group:
ABC Amber Winmail Converter is a useful tool which reads TNEF files (winmail.dat files) and extracts attachments (MS Office Outlook doesn't need to be installed) and RTF body to any document format. The software supports a run from a command line and more than 50 languages.
As all "ABC Amber" products, it's really easy as ABC and powerful as Amber.
ABC Amber Winmail Converter is freeware. This means that the program can be freely used at no charge.