ZenTest provides 4 different tools and 1 library: zentest, unit_diff,
autotest, multiruby, and Test::Rails.

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.

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.

Test::Rails helps you build industrial-strength Rails code.