An excellent (and free) Eclipse plugin called EclEmma was recently released that is well worth having a look at.
EclEmma is a code coverage tool that measures exactly how much of your codebase is being executed when you run your application. Typically, you'd use it to measure how much coverage your unit tests are getting, but it's not limited to just tests. You could just as easily use it to perform usability testing for a GUI application to find areas of your application that aren't being used.
The integration with Eclipse is very nice. You can select exactly which code you want to cover (although there are few limitations around this) and the end result is a nice navigable list of source files showing percentage covered for each method. It highlights your source code too so you can see exactly which lines are (and probably more importantly those that are not) being executed.
Behind the scenes, EclEmma is based on the popular open source code coverage tool Emma (although you probably already guessed that from the name) which has been around for quite some time and is rock solid. Emma is also the tool of choice for the new code coverage functionality in version 6 of IntelliJ IDEA.
One of the unfortunate side-effects of code coverage is that it can be extremely frustrating! It's all too tempting to keep hammering away at your unit tests to try and reach that elusive 100% coverage. A word of advice... don't bother! You can waste many an hour doing that, time that's better spent writing real code :)
