SyntaxHighlighter helps a developer/coder to post code snippets online with ease and have it look pretty. It’s 100% Javascript based and it doesn’t care what you have on your server. It supports multiple languages and is very lightweight, compressed core library is 11kb plus whatever brushes you need. However, SyntaxHighlighter isn’t for those looking for ability to edit highlighted code. SyntaxHighlighter doesn’t allow users to edit the source.

Requirements: Modern Browsers with Javascript Enabled
Demo: http://code.google.com/p/syntaxhighlighter/
License: LGPL License