The ThOR project is an API that enables developers to record the state or behavior of live Java objects for playback during unit testing. Recordings take the form of Java factory classes that can be used within a unit test to rehydrate complex object state or emulate complex object behavior.
Integration tests that connect to external systems such as databases or web services can quickly be converted to unit tests by recording the interaction with the external system at the interface. Check out the documentation on the ThOR project site.
There is also a free online book covering additional techniques to help developers rapidly increase unit test coverage on an existing project.