Use a third-party CSS or JS Framework, a self-hosted service like a CRM, or a third-party service with the site.
PayPal Reports module enables you to save PayPal transactions and analyse it later.
The Convio API module implements the Convio Open APIs to integrate Convio's back-end services with Drupal.
Mailchimp Back Sync bring extra synchronisation functionalities that are not available on the Drupal Mailchimp module version 7.x-3.x.
This module provides Drupal API integration for the excellent FullCalendar jQuery plugin.