Microsoft Windows SharePoint Services 3.0 is available as a free add-on for Windows Server 2008.  

SharePoint is a web-based application that helps take Microsoft Office beyond the desktop application, bringing it online to allow more effective sharing or collaboration within your company.

• Improve team productivity with easy-to-use collaborative tools
  Connect people with the information and resources they need. Users can create team workspaces, coordinate calendars, organize documents, and receive important notifications and updates through communication features including announcements and alerts, as well as the new templates for creating blogs and wikis. While mobile, users can take advantage of convenient offline synchronization capabilities.
• Easily manage documents and help ensure integrity of content
  With enhanced document management capabilities including the option to activate required document checkout before editing, the ability to view revisions to documents and restore to previous versions, and the control to set document- and item-level security, Windows SharePoint Services can help ensure the integrity of documents stored on team sites.
• Deploy solutions tailored to your business processes
  While standard workspaces in Windows SharePoint Services are easy to implement, organizations seeking a more customized deployment can get started quickly with application templates for addressing specific business processes or sets of tasks.
• Build a collaboration environment quickly and easily
  Easy to manage and easy to scale, Windows SharePoint Services enables IT departments to deploy a collaborative environment with minimal administrative time and effort, from simple, single-server configurations to more robust enterprise configurations. Because deployment settings can be flexibly changed, less pre-planning time is required and companies can get started even faster.
• Reduce the complexity of securing business information
  Windows SharePoint Services provides IT with advanced administrative controls for increasing the security of information resources, while decreasing cost and complexity associated with site provisioning, site management, and support. Take advantage of better controls for site life-cycle management, site memberships and permissions, and storage limits.
• Provide sophisticated controls for securing company resources
  IT departments can now set permissions as deep down as the document or item level, and site managers, teams, and other work groups can initiate self-service collaborative workspaces and tasks within these preset parameters. New features enable IT to set top-down policies for better content recovery and users, groups, and team workspace site administration.
• Take file sharing to a new level with robust storage capabilities
  Windows SharePoint Services supplies workspaces with document storage and retrieval features, including check-in/check-out functionality, version history, custom metadata, and customizable views. New features in Windows SharePoint Services include enhanced recycle bin functionality for easier recovery of content and improved backup and restoration.
  Easily scale your collaboration solution to meet business needs    Easily scale your collaboration solution to meet business needs.
  
  (source: Microsoft Technet)