Allan Collins

This plugin will allow you to add code snippets to your pages/posts. This is great for code tutorial sites.
<h3>Upgrading</h3>
If you are upgrading from version 1.0, you will see an option on the Code Library page that will allow you to upgrade. As a precautionary measure, be sure to back up your database before doing so.

WordPress Code Snippet

This plugin will allow you to add code snippets to your pages/posts.  This is great for code tutorial sites.

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.

WordPress Code Snippet vs WP-Markdown