Prism is a lightweight, extensible syntax highlighter, built with modern web standards in mind.
jQuery JSONView is a plugin to view JSON in a more readable format. It offers some methods and options to customize.
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.
jQuery Litelighter is a lightweight yet powerful syntax highlighting plugin. It comes with light and dark themes and js, html, css languages by default.
markItUp! is a very lightweight, customizable and flexible engine made to meet the developer's needs in their CMSes, blogs, forums or websites.