If you ever wanted to see how validity of DKIM signature will be affected by changing email headers and / or message body
There's an easy online tool for that now!
Well, as long as pasting text into a webform is easier for you than into a locally installed opendkim-testmsg CLI tool.
Just navigate to opendkim-testmsg.shpakovsky.ru and play with your message. Possible uses include restoring messages mangled by mailing lists into their "original" state.
Let me know if you find any use for it :)