For Java developers that also use eclipse, try this plugin:
EclemmaWhat is it? A code coverage tool for junit test cases. Ever write some test cases, then not have a clue what is tested and what isn't? Run eclemma. Its the open source coverage tool 'emma', but once the test finishes, it uses highlighter pens in eclipse to show you what was tested, and what wasn't. Made my project go from 24% total code coverage to over 60%. Impressive tool that does the job, if you do unit tests (like you should), or, even better, use test-driven development.
Best of all, when you get it and people ask you what it is, you reply nonchalantly, "I took a vacation in Tawain and came back with the eclemma" (sounds like a disease, eh?)