This presentation: http://www.cafeconleche.org/slides/stpcon2005/codecoverage/
Cobertura: http://cobertura.sourceforge.net/
Measure test coverage with Cobertura: http://www.ibm.com/developerworks/java/library/j-cobertura/
Jester: http://jester.sourceforge.net
Test your tests with Jester: http://www.ibm.com/developerworks/library/j-jester/?ca=drs-j1105
Clover: http://www.cenqua.com/clover/