Sharepoint 2007: SPBuiltInFieldId a Class You Must Know

A short comment today about a class I've just discovered: SPBuiltInFieldId With this class you have access to all built-in fields, even the hidden one's Example:

SPBuiltInFieldId.WorkflowListId SPBuiltInFieldId.WorkflowItemId SPBuiltInFieldId.WorkflowVersion SPBuiltInFieldId._CopySource SPBuiltInFieldId._RightsManagement

Link: http://msdn2.microsoft.com/En-US/library/microsoft.sharepoint.spbuiltinfieldid_members.aspx