Most popular Mocking framework for unit tests written in Java
...Fancy getting world-wide visibility and building up an eternal fame of an OSS contributor? Use the latest version! Hack and experiment. Speak up at the mailing list. Mockito is a mocking framework that tastes really good. It lets you write beautiful tests with a clean & simple API. Mockito doesn’t give you hangover because the tests are very readable and they produce clean verification errors.
"Yet Another Do-It-Yourself (Test) Automation Framework" specification. Use or modify one of the existing software implementations (hosted/referenced here), or implement the framework yourself. Download and read the spec for more details.