Clover

Clover is a Java tool for generating code coverage reports from unit tests. It can be executed as a plugin in Ant, Eclipse, or Maven. It can also use to collect coverage data of integration tests.

Features:
     1. Open source and free to use tool for Java
     2. Very easy to use because of on-the-fly byte code
     3. Inline annotation helps to visually highlight statement-level coverage
     4. Test optimization feature makes it painless to run testsuite before every check-in

More Information and Official Website:
Download : https://www.atlassian.com/software/clover/download/