Can I use Team Foundation Server alone without buying Visual Studio Team System?
Yes. In order to do this you will need to buy Visual Studio Team Foundation Server and a Client Access License for proper communication with the server. Team Explorer, which is included with Team Foundation Server, is a stand alone client that uses the Visual Studio 2005 shell providing a UI for source control, work item tracking and all the other features. For more information on licensing Visual Studio Team System (including Team Foundation Server) see the Team System Licensing White Paper.