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.

garethgillman

This plugin prettifies/beautifies/highlights source code placed inside <code><pre class="prettyprint"></code> tags. It is a light plugin with only 2 files.
New in 3.2.2 – use the hammer icon in the editor to paste your code and automatically have it converted to pretty code.

Syntax Highlighter Lite

This plugin prettifies/beautifies/highlights source code placed inside `` tags. It is a light plugin with only 2 files.

WP-Markdown vs Syntax Highlighter Lite