UML virtual machines are networkable, to each other, to the host, and to other physical machines. So, UML can be used to set up a virtual network that allows setting up and testing of experimental services. See the networking tutorial and virtual network screenshot for more information.

David Coulson is running an entire network of UMLs, the public face of which is usermodelinux.org, which is running in one of them. He has written an paper on his adventures here. He also has a nice UML monitoring and control daemon which we intend to steal and use for umlcoop.org. Information about that can be found here.