You can edit Markdown, along with other types of wiki markup, directly in Eclipse. Here’s a screenshot of the Markdown editor in action:
Note the “Preview” tab which will show you what the page looks when rendered.
This support is provided by the Mylyn Docs project’s WikiText component. You can also edit Confluence, MediaWiki, Textile, TracWiki, and TWiki formats. There’s a patch in the works to add AsciiDoc support as well.
WikiText is included in the Eclipse IDE for Java Developers and other packages.