EMMA
EMMA is an open-source toolkit for measuring and reporting Java code coverage. EMMA distinguishes itself from other tools by going after a unique feature combination: support for large-scale enterprise software development while keeping individual developer's work fast and iterative. Every developer on your team can now get code coverage for free and they can get it fast!

GroboCodeCoverage
There are several commercially available code coverage tools for Java, but they all require a large fee to use. This is a 100% Pure Java implementation of a Code Coverage tool. It uses Jakarta's BCEL platform to post-compile class files to add logging statements for tracking coverage.

Hansel
Hansel is an extension to JUnit that adds code coverage testing to the testing framework.

InsECT
which stands for Instrumentation Execution Coverage Tool, is a system developed in Java to obtain coverage information for Java programs. InsECT instruments (inserts instructions into) Java class files at the bytecode level with probes to report information about a system at runtime.

JBlanket
JBlanket is a tool for assessing and improving method coverage of unit test cases. It is integrated with JUnit and Ant.

jcoverage/gpl
jcoverage/gpl is a free code-coverage tool for Java™ programmers that allows them to measure the effectiveness of their Java tests and how much of a software program's code has been tested.

Jester
Jester finds code that is not covered by tests. Jester makes some change to your code, runs your tests, and if the tests pass Jester displays a message saying what it changed. Jester includes a script for generating web pages that show the changes made that did not cause the tests to fail.

JVMDI Code Coverage Analyser
This small utility is a shared library which when loaded into a Java VM (1.4+) which supports JVMDI will record all the lines of code executed. This is a relatively coarse coverage method, but good enough for a lot of purposes.

NoUnit
NoUnit: Gives you clear pictures of your code and tests (Green is good , Red is bad!). Is a 100% Java and XML solution , freely available under the GNU License. Can be extended to show any code pictures you want (Any view of your code , not just JUnit tests!). Reports can be understood by anybody , including non-techies :-)

Quilt
Quilt is a Java software development tool that measures coverage , the extent to which unit testing exercises the software under test. It is optimized for use with the JUnit unit test package, the Ant Java build facility, and the Maven project management toolkit.