From Omicron Software Systems:
Mail to ToDo X is an AppleScript to connect Apple's Mail with the shareware to-do list application ToDo X. It provides a convenient way to create to-do items from selected email messages.
The to-do item's name will be the message subject, and the notes will contain either an excerpt of the message or (if you prefer) its full text. At the end of the notes you'll find a clickable link that will open the message in Mail.