my_app.rb

require 'sinatra/base'

class MyApp < Sinatra::Base
  @@my_app = {}
  def self.new(*) self < MyApp ? super : Rack::URLMap.new(@@my_app) end
  def self.map(url) @@my_app[url] = self end

  class FooController < MyApp
    map '/foo'

    get '/' do
      "foo"
    end
  end

  class BarController < MyApp
    map '/bar'

    get '/' do
      "bar"
    end
  end

  run! if __FILE__ == $0
end

Totally awesome! Thank you for this.

I built a more fleshed out application and used this code for controllers. I'm sharing a skeleton of it here: https://github.com/jfeaver/modular-sinatra