Drupal integration with other applications

Drupal being a top CMS, its very important that it integrates with other third party applications. We generally want the following

  1. Use and implement some standard functionality of Drupal
  2. Build some custom functionality that should seamlessly integrate with Drupal eg same users, same permissions, etc...
  3. Use Third party modules for front end or implementing key functionalities. 
The most frequent asked questions relate to:
  1. Sharing a user base
  2. Sharing authentication
  3. Synchronizing a dual user base
  4. External authentication
  5. Session handler issues
  6. Theme engine integration
  7. Import scripts
Some tips may be unique to a third party application, and some may be reusable by several applications. Check the following pages on Drupal.org

Modules for third party integration

Drupal has a suite of modules for almost all major Third party integration. Check out the category here