Mocha is a library for mocking and stubbing using a syntax like that of JMock.
It can be used with many testing frameworks e.g. Test::Unit, RSpec, test/spec, expectations, Dust, MiniTest and even JtestR.
Mocha provides a unified, simple and readable syntax for both traditional mocking and partial mocking.
The documentation can be found at here.
The sourcecode is available on Github.