Markdown is John Gruber's text markup langauge and the Perl program that converts documents written in that language into HTML. CL-Markdown is a Common Lisp rewrite of Markdown. CL-Markdown is licensed under the MIT license.
You can see the source of this page by clicking in the address bar of your browser and changing the extension from html to text. For example, this page's source is at index.text.
You can view a comparison of Markdown and CL-Markdown output here.
Thanks to Common-Lisp.net, CL-Markdown is set up with a Trac. If you've tried it, you may want to add a ticket.
The easiest way to get setup with CL-Markdown is by using ASDF-Install. If that doesn't float your boat, there is a handy gzipped tar file and a Darcs repository. The darcs commands to retrieve the CL-Markdown source is:
darcs get "http://common-lisp.net/project/cl-markdown"
(note that this won't let you build CL-Markdown unless you also get all of its dependencies...)