Several years ago, I found that differential code coverage was extremely powerful as a debugging tool and made a prototype using EclEmma. I also had some communications with the EclEmma team, and put in a feature request for differential code coverage. Actually, I thought that incremental code coverage would be easier for people to understand and use.
Well, EclEmma 2.1 just came out, and it has incremental code coverage in it! I am really excited by this, and wrote a blog post at my workplace. I have given motivation on this blog before, but I give some more on the Tasktop blog posting, as well as some instructions for which buttons to push to effectively use EclEmma to do incremental code coverage.