Use a third-party CSS or JS Framework, a self-hosted service like a CRM, or a third-party service with the site.
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.
Slack is amazing.