Sau Syntax is the easiest way to post source code on your blog posts.
Plugin to customize the display of the pre tag in WordPress. Easily select a theme for the pre tag and click apply. No need to add any additional info
Shortcode implementation of Codeflask
The best Google Code Prettify WordPress plugin.
Gives you the ability to add Github Gists to your site decorated with line numbes and alternating line color.