Paws For A Cause had me build them a highly customized Drupal 7 site that serves as an e-commerce platform, implementing Drupal Commerce and several custom modules.
One project must-have was HTML emails, which Drupal doesn't excel at by any means. This led me to implement the Mandrill API, a product by MailChimp, which brings much more than pretty emails to the table (i.e. analytics).
To augment the shopping experience and hold true to their slogan, we also implemented our own pet adoption search which leverages the Petfinder API.