JUnit: The Spring Framework's unit and integration test suite, written with JUnit 3.8.2 and JUnit 4.7 as the testing framework.

TestNG: A testing framework inspired by JUnit 3.8 with added support for Java 5 annotations, test groups, data-driven testing, distributed testing, and so on.

MockObjects.com: Web site dedicated to mock objects, a technique for improving the design of code within test-driven development.

"Mock Objects": Article in Wikipedia.

EasyMock: Used extensively by the Spring Framework in its test suite.

JMock: Library that supports test-driven development of Java code with mock objects.

Mockito: Java mock library based on the test spy pattern.

DbUnit: JUnit extension (also usable with Ant and Maven) targeted for database-driven projects that, among other things, puts your database into a known state between test runs.

Grinder: Java load testing framework.