Remark parses HTML and delivers proper Markdown.
$ [sudo] gem install remark
From command-line:
remark path/to/file.html
or by STDIN:
echo "..." | remark
You can try feeding it a document from the web:
curl -s daringfireball.net/projects/markdown/basics | remark > result.markdown
See how it does.
If you've cloned the repository, invoke the binary like this:
ruby -Ilib -rubygems bin/remark spec/sample.html
And this is how you use it from Ruby code:
Remark.new('<h1>My document</h1><p>Some content</p>').to_markdown