We posted a detailed case study about how Server Check.in was built on Drupal.org: Server Check.in case study.

From the case study:

Why Drupal was chosen:
Drupal provides an extremely robust and flexible platform for building websites. Server Check.in was built to be simple, inexpensive, and easy to use. More than half of the site's features were simply pieced together using modules already built by others. Missing parts—most of the backend of the service, along with payment and notification integration—were easy to add through a few custom modules.

Describe the project (goals, requirements and outcome):
Server Check.in was built using Drupal 7, and the responsive theme was built using a Zen sub-theme. Drush powers most of the backend functionality of the service (server checks, queue consumption, notification handling). Some parts of the service are provided through third party services: Stripe powers payments, and Twilio powers SMS notifications.

Read the entire case study. (Earlier, I also posted a thread with more details about Server Check.in on Hacker News).