I use SyntaxHighlighter Evolved on my site which is based on Alex Gorbatchev’s SyntaxHighlighter and used by WordPress.com blogs/sites to get something like this:
And i just recently published a plugin named Simple Gist Embed
This Plugin allows to embed GitHub Gist to your posts/pages/custom
using a simple shortcode and even simpler using a built-in Tinymce
button. I know there are other plugins which do that already but this
plugin also lets you create a GitHub Gist From within your WordPress
admin. Other then the option to create Gists from your admin this
plugin is faster then the others because of it’s built-in caching of
Gists to the database.