It comes from: http://commonservicelocator.codeplex.com/
From the project description:
The Common Service Locator library
contains a shared interface for
service location which application and
framework developers can reference.
The library provides an abstraction
over IoC containers and service
locators. Using the library allows an
application to indirectly access the
capabilities without relying on hard
references. The hope is that using
this library, third-party applications
and frameworks can begin to leverage
IoC/Service Location without tying
themselves down to a specific
implementation.
This abstraction is implemented by several IoC frameworks out there, and Unity is one of them. nevertheless, Unity is not the owner of that project.
This way StructureMap, for example, provides implementation of those clases, so other frameworks (such as Prism from patterns & practices) can easily work with other IoC frameworks and not be bound to Unity itself.