Instructions on how to use this script can be found at 
http://walkingthestack.wordpress.com/2010/09/21/importing-a-tfs-repository-into-a-git-repository-using-powershell/

Available parameters for this script:

-TFSRepository (mandatory)
The TFS repository you wish to import. Ie. $/repository. 

-Server (optional)
The URL for your TFS server ex: https://tfs.acme.com/

-GitRepository (optional)
The name you want to give the Git repository. If no parameter is supplied, the default name will 'ConvertedFromTFS'.

-WorkSpaceName (optional)
During the import a temporary TFS workspace will be created. You can specify the name you wish to use for this workspace. Default 'TFS2GIT'.

-StartingCommit (optional)
You can import a sequential range of commits. Specify the first TFS commit you want to import.
When this parameter is used, EndingCommit must also be used.

-EndingCommit (optional)
You can import a sequential range of commits. Specify the last TFS commit you want to import.
When this parameter is used, StartingCommit must also be used.

-UserMappingFile (optional)
The script can convert TFS user accounts to Git accounts. For this to work you need to specify a user mapping file.
See the content of 'usermappings.txt' for an example.

-t (optional)
creates a lightweight tag on the first and last imported commits 

Examples:
Converting a TFS repository with full history

.\tfs2git "$/TFS/repository/path" https://tfs.yourserver.com/ -UserMappingFile userMappings.txt

or

.\tfs2git -TFSRepository "$/TFS/repository/path"


Converting a TFS repository with a limited history

.\tfs2git -TFSRepository "$/TFS/repository/path" -StartingCommit 1 -EndingCommit 100


Converting a TFS repository while using a usermapping file

.\tfs2git -TFSRepository "$/TFS/repository/path" -UserMappingfile filename