The Transformation web service
This service exposes the ESB feature of messages transformation. It does so without needing to persist the actual message into the message box to do it.
It has one single operation named Transform that takes as an input the XML message to be transformed and the fully qualified name of the map deployed in BizTalk. It returns the XML representation of the transformed message.
Systems consuming this service shouldn't necessarily need to know the format in which the maps' names are qualified in BizTalk, so it could be useful to use the Resolver service to resolve the name of the map, and then use the output of that resolution as an input for the fully qualified name of the map on the Transform call.
This service is also available in both ASMX and WCF flavors.