Drupal is defined as a content management system in first instance by many, but lately it has been used as a framework for the development of web applications. Such applications range from a repository of documents between users, an Intranet to share information (for example Open Atrium which is a free software project based on Drupal) to an application for managing technology investments as IT Dashboard.
Using Drupal as a web application is thanks to the use and integration between contributed modules and now with Drupal 7 there is the ability to scale because the modules can now benefit from module entities which allows you to manage the structure the content. In this way you can set properties for each entity and allows the extensibility of the modules on Drupal. For example Commerce Drupal (E-commerce module) and Profile 2 (module to create user profiles).
To find out more about Entities you may enter this post. In Rootstack we have been developing interesting projects such as social networks, repositories of information with complex content structures and sites of e-commerce among others.
The ease of using Drupal and contributed modules like Organic Groups, Search API and User Relationships have given us the flexibility to create custom applications to meet our customers' expectations. What web application have you developed using Drupal? We would like to read your comments and points of view.