app.rb

require 'rubygems'
require 'bundler'
Bundler.setup

require 'active_support/core_ext/class/subclasses'
require 'sinatra/base'

module Keister
  class App < Sinatra::Base
    # settings

  end
end

app_dir = File.expand_path(File.join(File.dirname(__FILE__), 'app'))

Dir[File.join(app_dir, 'controllers', '*.rb')].each do |controller|
  require controller
end

module Keister
  def self.app
    @app || Rack::Builder.new do
      Keister::App.descendents.each do |subapp|
        use subapp
      end
      run Keister::App
    end
  end

end

my_controller.rb

class Keister::MyController < Keister::App

  get '/' do
    'Hello, world!'
  end

end