Good groupware service?

Please tell me a good service for collaboration.


What it is:

• Creating and tracking tasks.
  
• Discussion of the tasks.
  
• The exchange and storage of documents.
  
• Storage of internal documents and rules in the wiki format.
  

Additionally it would be nice to have the following functions:

• A list of contacts.
  
• Calendar.
  
• The hierarchy of participants (Manager, contractor).
  
• Subtasks.
  

Requirements:

• Free.
  
• The ability to install on the server.
  
• Easy.
  
• Convenience.
  

At the moment the main candidate is Remain. But perhaps there is a more simple functionality of the service.