Stephen Harris

This plugin allows you to write posts (of any post type) using the Markdown syntax. The plugin converts the Markdown into HTML prior to saving the post. When editing a post, the plugin converts it back into Markdown syntax.
The plugin also allows you to enable Markdown in comments and bbPress forums. In these instances the plugin adds a toolbar, and preview of the processed Markdown with <a href="http://code.google.com/p/google-code-prettify/" rel="nofollow ugc">Prettify</a> syntax highlighter applied (similiar to that used in the Stack Exchange websites such as <a href="http://wordpress.stackexchange.com/" rel="nofollow ugc">WordPress Stack Exchange</a>).
WP-Markdown stores the processed HTML, so deactivating the plugin will not affect your posts, comments or bbPress forums.

WP-Markdown

Version 1.5.1 of the plugin works almost perfectly.
I'm just waiting for the upcoming 1.6 update (which is in alpha on GitHub) to be pushed, so I can use GitHub Flavor-style codeblocks specifying the exact language.



Waiting for 1.6

Allows Markdown to be enabled in posts, comments and bbPress forums.

Fredrik Stigsson

This plugin uses <a href="https://github.com/googlearchive/code-prettify" rel="nofollow ugc">Google Code Prettify</a> with customized styling. This plugin uses the default skin, custom styling has been applied to show line numbers and to prevent line-breaks. The wp-code-block is styled by the theme (borders, background color and more), the contents of the pre-block is styled by css in the plugin.

Annytab Code Prettify

A lightweight plugin that automatically prettifies code in [pre] tags, with linenumbers and without line-breaks.

WP-Markdown vs Annytab Code Prettify