JUnit

From APIDesign

JUnit is the original testing harness for Java which started the whole XP movement. For a while it stagnated on version three and was almost superceeded by TestNG, but with the release of JUnit4 most of the people who already had an investment in JUnit tests decided to stay. In spite JUnit project sticks to obsolete license.

The situation may change now, when TestNG has excellent support for mixed execution of all kinds of test including JUnit3 and JUnit4 ones.