What is an easy way to stub / dummy a restful web service?

I've found using Sinatra really useful for this sort of thing if you want to test the actual HTTP calling code. You can have a endpoint returning data in seconds. Very little Ruby knowledge required.

require 'sinatra'
require 'json'

get '/Person' do
    content_type :json
    { :id => 345, :key2 => 'John Doe' }.to_json
end

Is all you would need to return a simple json object.

Mocky.io allows you to create stub endpoints and specify the data they return via public URLs.

Runscope (disclaimer, I'm a founder) allows you to capture a real request once, then replay back the response as needed via Response Playback URLs.