You can use Prot-On REST services to develop your own applications. In order to access them you will need to request us an API Key.
Most of these services will require you OAuth 2 authentication. The authentication process may differ depending on the programming language your application is built. You can find some JAVA and PHP examples on how to authenticate and make requests to the REST Api among this developers website.
You can get more information about the OAuth 2.0 protocol in its official site.
You can develop applications based on Prot-On features for most of the main cloud and content management systems.
I.e.: Check out this Prot-On Integration with Drupal.