How to setup an Intranet site and a FREE alternative to Sharepoint
Solution 1:
Hey, SharePoint Services (WSS) IS free. Microsoft Office Sharepoit Services (MOSS) isn't, but for the purpose of an intranet site, WSS should do it.
This is of course if you are using Windwos Server as a base... (which is not free).
Edit: Here is a good comparison between WSS and MOSS.
Solution 2:
I would recommend Mindtouch (formerly Deki Wiki). http://developer.mindtouch.com/
We use it for exactly this purpose; company announcements, sharing of public files and documents, and a knowledgebase and documentation repository for IT staff.
You can run it in the pre-provided VM image, which we are currently doing, or you can install source to a Linux server, or MSI to a Windows server.
You mentioned calendar features, which this won't currently do. However there has been talk of implementing Exchange Calendar access, its just no one from the community has stepped up to make it happen.
The open source verison is free, while you can pay for support and additional enterprise-level extensions (such as salesforce integration etc).
Solution 3:
Have a look at Alfresco open source Enterprise Content Management. It can be used standalone and integrates with Joomla and Drupal.