Applications on GOV.UK
The publishing platform of GOV.UK consists of at least 54 separate applications. Most of them are built using Ruby on Rails.
You can use the chrome extension to find out which application is rendering any given page.
The apps are secured by behind a single signon system. They use an omniauth adapter called gds-sso to authenticate the user. The styling of the apps are shared using a gem called govuk_admin_template.