example.rb

module Foo
  module String
    def to_s
      super + "zilla"
    end
  end
end

use Foo::String for String
  puts "God".to_s
end

#=> "Godzilla"

# merb-core/core_ext/string.rb

module Merb
  module String
    def camel_case
      return self if self !~ /_/ && self =~ /[A-Z]+.*/
      split('_').map{|e| e.capitalize}.join
    end
  end
end

# activesupport/core_ext/string.rb

module Rails
  module String
    def camel_case(first_letter = :upper)
      case first_letter
        when :upper then Inflector.camelize(self, true)
        when :lower then Inflector.camelize(self, false)
      end
    end
  end
end

# my_app.rb

"Hello".camel_case #=> NoMethodError

# my_app2.rb

use Merb::*
  "hello_goodbye".camel_case #=> "HelloGoodbye"
  "hello_goodbye".camel_case(true) #=> ArgumentError
end

use Merb::String for String
  "hello_goodbye".camel_case #=> "HelloGoodbye"
  "hello_goodbye".camel_case(true) #=> ArgumentError
end

# my_app3.rb

use Rails::*
  "hello_goodbye".camel_case #=> "HelloGoodbye"
  "hello_goodbye".camel_case(true) #=> nil
  "hello_goodbye".camel_case(:lower) #=> "helloGoodbye"
end

use Rails::String for String
  "hello_goodbye".camel_case #=> "HelloGoodbye"
  "hello_goodbye".camel_case(true) #=> nil
  "hello_goodbye".camel_case(:lower) #=> "helloGoodbye"
end