If I want to print a markdown file from GitHub as it appears on screen, for example: https://github.com/RestKit/RestKit/blob/master/Docs/Object%20Mapping.md
Then how can
http://github.com/github/markup
You don't mention a language preference, but this is the Ruby library GitHub itself uses to render the markup files. It has the advantage of supporting the various markups GitHub supports (textile, rdoc etc). Rendering .markdown files has a dependency on the redcarpet markdown parser:
http://github.com/tanoku/redcarpet