Free is TFS hosted on Windows Azure: http://tfspreview.com/
If you need more info about TFSPreview, please read Brian Harry's MSDN blog post: http://blogs.msdn.com/b/bharry/archive/2011/09/14/team-foundation-server-on-windows-azure.aspx
To obtain activation code just register there or contact someone from MS ALM team.
Update: TFS Preview goes live&stable as Visual Studio Online here: http://www.visualstudio.com still free for 5 team members and build server computing time. Another nice feature automatic build&deploy (daily or continuous integration) to Azure. More info: http://azure.microsoft.com/en-us/documentation/articles/cloud-services-continuous-delivery-use-vso/