Xmldiff is a utility for extracting differences between two xml files. It returns a set of primitives to apply on source tree to obtain the destination tree.
LogiLab's Xmldiff is interesting for several reasons, including the fact that it uses XUpdate to represent the XMl differences. You can then use 4Suite's command-line XUpdate tool (or any other tool you like) to "patch" XML files with the diff. See Sylvain Thénault's announcement.
Nice. Preliminary inspection seems to recommend it as a good example of 4XSLT in server architecture in general. It makes good use of the API, and even implements processor object pooling (helps performance). As the CherryPy tutorial says,
A filter is an object that has a chance to work on a request as it goes through the usual CherryPy processing chain.