highlight.js by Ivan Sagalaev.
Highlight.js highlights syntax in code examples on blogs, forums and in fact on any web pages. It's very easy to use because it works automatically: finds blocks of code, detects a language, highlights it.
This is not only convenient but also allows highlighting for code examples marked up with writing syntaxes like Markdown where there is no way to specify HTML class easily.
The library knows 41 languages and is bundled with 16 style themes.