I was the lead on the E-Z Rent A Car site redesign and development project. I worked with a team of designers and developers to completely redesign and build their site (including fully functional booking system) using Drupal 7.
This project involved a lot of custom development- ranging from the reservation system to marketing campaign functionality.
Migrations were also a huge portion of this project. All of their old data needed to be migrated from the existing database and re-architected to fit within Drupal. One of the migration highlights was getting hundreds of thousands of existing users migrated in a timely fashion via a custom Drush script.
You can read the case study for this project over at Designzillas.