The great thing about Drupal and other open source software is that it is relatively easy to integrate the platform with external software packages. This includes both proprietary software packages and open source packages. With the source code available for review, engineers can easily find ports for integrating open source solutions with other systems. For Drupal, I find that there has been a great deal of demand in integrating it with the following systems:
1) SalesForce.com: most mid to large size corporations that we work with use SalesForce.com as their CRM solution. Even though SalesForce.com is a hosted application, it has a great API which is well documented. In fact, there is a contributed Drupal module which allows for information be passed between SalesForce.com and Drupal. The module can be found here: http://drupal.org/project/salesforce. We have used an altered version of this module for Drupal and it has been working great.
2) Alfresco: while Drupal is a great content management system, Alfresco is an excellent document management system. It is great for building intranets and file repositories. By integrating the 2 systems together, an organization can easily manage both the web site and the intranet seamlessly. The module which integrates Alfresco and Drupal is still at an early stage and can be found here: http://drupal.org/project/cmis_alfresco. However, I see a lot of potential with this module as enterprises become more open to open source document management system. Currently most of these users are still big fans of Sharepoint.
3) SugarCRM: not all companies can afford the prices of SalesForce.com. For some enterprises they would rather have a hosted in-house CRM solution. This is where SugarCRM comes in a lot of times. I have personally used both of these solutions and do not find one to be much more superior to the other. Drupal has a contributed module which appears to provide some integration abilities: http://www.sugarforge.org/projects/sugarondrupal. (note: my team has never used this module, so we are not 100% sure about its effectiveness)
4) UserPlane: recently, we have been working on a few E-Learning related projects. We came to discovery of a system known as UserPlane. UserPlane is a web conferencing tool.
5) FB Connect: ever since about 2 months ago, a majority of the social network sites that we develop has the FaceBook Connect plugin built into the site. The Drupal FB Connect module has been working really well for us. Feel free to comment about your experiences with these integrations or other Drupal integration projects.