console.coffee

$ ->
  return if $("#console").length < 1

  $("#console_input").keyup (event) ->
    if event.which == 13
      command = $(this).val()
      $.ajax
        url: '/console'
        type: 'POST'
        dataType: 'text'
        data:
          cmd: command
        success: (data) ->
          cmd = $("<div class='cmd'></div>").text(command)
          out = $("<div class='out'></div>").text(data)

          $("#console .output").append(cmd).append(out)
          $("#console_input").val('')

console.haml

#console
  .output
  .input
    %input#console_input{type:"text", autofocus:true}

console.sass

#console
  background-color: #333
  color: #fff
  padding: 10px

  .output
    height: 20em

    .cmd
      color: #999

#console_input
  padding: 5px
  margin: 0
  background-color: #666
  color: #fff
  border: none
  height: 1em
  width: 740px

console_controller.rb

class ConsoleController < ApplicationController

  def index
    if params[:cmd]
      result = begin
        $repl ||= REPL.new

        result = $repl.instance_eval <<-EOF.trim_gutter
          | result = (#{params['cmd']})
          | @_#{current_employee.id} ||= {}
          | @_#{current_employee.id}.update(local_variables.inject({}) do |h, v|
          |   h.update({v => eval(v.to_s)})
          | end)
          | result
        EOF

        result.inspect
      rescue=>e
        "Error: " + e.message
      end
      render :text => result
    end
  end

  class REPL

    def method_missing(method, *args, &block)
      eval <<-EOF.trim_gutter
        | @_#{current_employee.id} ||= {}
        | @_#{current_employee.id}[method.to_sym] || super(method, *args, &block)
      EOF
    end

    def reload!
      $repl = REPL.new
      'ok'
    end
  end
end

routes.rb

match 'console' => 'console#index'