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.
Jester is different than code coverage tools, because it can find code that is executed by the running of tests but not actually tested. Jesters approach is called mutation testing. However, Jester is not meant as a replacement for code coverage tools, merely as a complementary approach.
Jester is for Java code and JUnit tests (download the latest version of Jester).