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, 1.9.3 or 2.0+. It also runs on JRuby.

require 'coderay'
html = CodeRay.scan("puts 'Hello, world!'", :ruby).div(:line_numbers => :table)

See rubydoc.