Prettify is the code colorizer that StackOverflow uses.

NOTE: This answer was originally posted at StackOverflow.com by Zach Johnson

You may want to look at SyntaxHighligher. It uses JavaScript so it’s not using your own server’s resources and supports a bunch of color schemes. http://alexgorbatchev.com/wiki/SyntaxHighlighter

The problem with putting your code in a generator on another website is that if you change it somewhere you have to put it back in, which can become tedious.

NOTE: This answer was originally posted at StackOverflow.com by aelx

CodeColorizer is one.

NOTE: This answer was originally posted at StackOverflow.com by SLaks

Do you mean code pasting services such as Pastie?

NOTE: This answer was originally posted at StackOverflow.com by Esko

I recently found http://hilite.me. I like it because there is an option for formatting text–useful when you are posting a config file or a series of commands.

From a WordPress standpoint I’ve been trying to trim back the number of plugins I’m running after a scan on pingdom.com showed how much time my page loads were taking because of a syntax highlighting plugin–and I rarely post code so it was a really not worth it.

I like the idea of inserting code blocks as pure html in the post because you aren’t required to have that plugin in the future and I’d assume won’t impact page load times.

NOTE: This answer was originally posted at StackOverflow.com by John Poelstra

I use WP-syntax for WordPress. Works great for me.
It’s not exactly what you are looking for but I thought I’d mention it here if anyone is looking for syntax hightlighting in WordPress.
From the usage description:

Wrap code blocks with <pre lang=”LANGUAGE” line=”1″> and </pre>
where LANGUAGE is a GeSHi supported
language syntax.
The line attribute is optional.

NOTE: This answer was originally posted at StackOverflow.com by windyjonas