ZenTest provides 4 different tools: zentest, unit_diff, autotest, and
multiruby.

ZenTest scans your target and unit-test code and writes your missing
code based on simple naming rules, enabling XP at a much quicker
pace. ZenTest only works with Ruby and Test::Unit. Nobody uses this
tool anymore but it is the package namesake, so it stays.

unit_diff is a command-line filter to diff expected results from
actual results and allow you to quickly see exactly what is wrong.

autotest is a continous testing facility meant to be used during
development. As soon as you save a file, autotest will run the
corresponding dependent tests.

multiruby runs anything you want on multiple versions of ruby. Great
for compatibility checking! Use multiruby_setup to manage your
installed versions.