Highlight.js highlights syntax in code examples on blogs, forums and, in fact, on any web page. It's very easy to use because it works automatically: finds blocks of code, detects a language, highlights it.

Features

71 languages and 44 styles
automatic language detection
multi-language code highlighting
available for node.js
works with any markup
compatible with any js framework

Rainbow is a code syntax highlighting library written in Javascript.
It was designed to be lightweight (1.4kb), easy to use, and extendable.
It is completely themable via CSS.