The Code Highlight plugin enables displaying computer code easily. This plugin renders character entities on the fly, so you can copy/paste normal code directly into your posts and it will always look like normal code, even when editing the post (i.e., no pre-processing of the code is required).
Include line numbers (customizable starting number). The best part about the line numbers - visitors can copy/paste the code from your post, leaving the line numbers behind!
Accepts BBcode tags and doesn't render smilies. Coloration of PHP code, plus PHP manual links for PHP functions. Easy to install and easy to use. No hacks. Degrades nicely, if the plugin is off. Styling completely customizable via an included CSS file.