I will add some small (and rather hackish) scripts for my personal use here.
footnotes will convert footnotes in MultiMarkDown format
to the format of the Wordpress plugin
footnotes and back.
Usage: footnotes --to mmd|wp [--file $inputfile]
mmm (MIME multipart/alternative from Markdown) can
create a MIME multipart/alternative mail, containing a
text/plain part (in Markdown) and a text/html part, from
a Markdown file. Other mail headers (From:, To:, Subject:, ...)
can be prepended from a template file.
Usage: mmm [--file $inputfile] [--headers $templatefile]