The Microsoft Dynamics CRM 4.0 Advanced Data Migration Map Editor was created by the Microsoft CRM IDC development team during testing of Microsoft Dynamics CRM 4.0 and is being made available to the CRM Community on CodePlex under the Microsoft Public License. 
The Map Editor allows users to prepare the advanced XML maps needed for migrating data into Microsoft Dynamics CRM 4.0 using Data Migration Manager. This tool enables the user to create and edit entity maps, attribute maps, complex transformation maps, pick list maps, reference maps and user maps. A user guide as well as the source file is provided at the link.