CodeRay is a Ruby library for syntax highlighting.
You put your code in, and you get it back colored; Keywords, strings, floats, comments - all in different colors. And with line numbers.
gem install coderay
CodeRay needs Ruby 1.8.7+ or 1.9.2+. It also runs on Rubinius and JRuby.
require 'coderay'
html = CodeRay.scan("puts 'Hello, world!'", :ruby).div(:line_numbers => :table)