Amiga............: YAM          http://www.yam.ch/
  Unix.............: Metamail     ftp://ftp.bellcore.com/nsb/
  Windows/Macintosh: Eudora       http://www.qualcomm.com/

General info about MIME can be found at:

http://www.cis.ohio-state.edu/hypertext/faq/usenet/mail/mime-faq/top.html