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.

lexn

This plugin has prettify.js copied directly from google-code source browser. So your code will look exactly like it does on google-code. 
CSS is slightly modified to embolden keywords and types.
<h3>License</h3>
See prettify.js for license details (Apache License 2.0). Other files are available under GPLv2.
<h3>Usage</h3>
<pre class="prettyprint"> 
 // Put your code here. 
</pre>
Or with shortcode:
[code] 
 // Put your code here. 
[/code]
<h3>Making Code HTML Safe</h3>
The plugin automatically makes highlighted code html-safe:
<pre class="prettyprint"> 
 Anything that goes here will be quoted to appear on the page as-is. 
 For example: <head></head><body></body> 
</pre>
To avoid quoting your code, add dontquote as the first class.
<pre class="dontquote prettyprint"> 
 Remember to quote manually. 
 For example: &lt;head&gt;&lt;/head&gt;&lt;body&gt;&lt;/body&gt; 
</pre>
<h3>Line Numbering</h3>
Line numbdering can be enabled as follows:
<pre class="prettyprint linenums"> 
 // Put your code here. 
</pre>
If you want to start with a line number 42, you can do the following:
<pre class="prettyprint linenums:42"> 
 // Put your code here. 
</pre>
<h3>Highlighting Lines</h3>
When line numbering is enabled you can highlight one or several lines of code using the highlight class:
<pre class="prettyprint lang-YOURLANG linenums highlight:2,4"> 
 // line 2 
 // line 3 
 // line 4 
</pre>
Alternatively you can highlight one or several code segments:
<pre class="prettyprint lang-YOURLANG linenums highlight:1-3,5-6"> 
 // line 2 
 // line 3 
 // line 4 
 // line 5 
</pre>
Note: This feature is not available when line numbering is disabled.
<h3>Specifying Language</h3>
Most of the time the highlighter will do a good job guessing how to highlight the code. However, to achieve the best result, you should specify the language:
<pre class="prettyprint lang-YOURLANG"> 
 // Put your code here. 
</pre>
Replace YOURLANG with one of the following:
<ul>
<li>aea</li>
<li>agc</li>
<li>apollo</li>
<li>bsh</li>
<li>c</li>
<li>cc</li>
<li>cl</li>
<li>cpp</li>
<li>cs</li>
<li>csh</li>
<li>css-str</li>
<li>cv</li>
<li>cxx</li>
<li>cyc</li>
<li>default-markup</li>
<li>el</li>
<li>fs</li>
<li>go</li>
<li>hs</li>
<li>htm</li>
<li>html</li>
<li>java</li>
<li>js</li>
<li>json</li>
<li>lisp</li>
<li>lua</li>
<li>m</li>
<li>ml</li>
<li>mxml</li>
<li>perl</li>
<li>pl</li>
<li>pm</li>
<li>proto</li>
<li>py</li>
<li>rb</li>
<li>scala</li>
<li>scm</li>
<li>sh</li>
<li>sql</li>
<li>vhd</li>
<li>vhdl</li>
<li>wiki</li>
<li>xhtml</li>
<li>xml</li>
<li>xsl</li>
<li>yaml</li>
<li>yml</li>
</ul>
<h3>Shortcode</h3>
Here is a quick example. All the parameters are optional.
[code lang=”scala” start=”1″ highlight=”1-3,6,10-14″ background=”#C0FFB3″] 
 // Put your code here. 
[/code]
Html quoting can be disabled as follows:
[code quote=”false”] 
 // Put your code here. 
[/code]
The contents of shortcodes is auto-formatted by WP. As a result and tags along 
with some formatting will be lost. If you need to preseve the original formatting use 
html notation instead of the shortcode.
<h3>Please Support</h3>
If you like Prettify GC Syntax Highlighter plugin, please donate a minute of your time and vote for it. Thank you!

Prettify GC Syntax Highlighter

Your code will look exactly like it does on google-code.

WP-Markdown vs Prettify GC Syntax Highlighter