Version 0.6.0 of the rcov code coverage tool for Ruby [ http://eigenclass.org/hiki.rb?rcov ] features a new differential code coverage mode which will tell you when you've added new code that was not covered by the tests. This release also includes a compiler plugin for vim (contributions for other editors/IDEs welcome).

Cross-referenced reports, which were recently introduced, have been expanded to indicate where methods are called from and which methods were called for each line. In addition to that, cross-referenced report generation is now over 4 times faster for applications with deep call stacks (such as Rails apps).

See http://eigenclass.org/hiki.rb?rcov+0.6.0 for more information.