I just lurk on this list so I may be wrong but as I understand it Nathan started one. (Lapidary I think) then when enough ppl were using RubyUnit I think he wanted to make them work together. And ended up creating UnitTest. I am a firm beliver in unit testing (junit and httpunit)and as soon as I understand Ruby enough to justify using it I'm looking forward to trying UnitTest. 

Carl Parrish
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From: John Carter <john.carter / tait.co.nz>
Date: 2002/05/15 Wed PM 06:53:30 EDT
To: ruby-talk / ruby-lang.org (ruby-talk ML)
Subject: The various UnitTest frameworks.

Looking in RAA I see three UnitTest frameworks...
  http://www.ruby-lang.org/en/raa-list.rhtml?name=Lapidary
  http://www.ruby-lang.org/en/raa-list.rhtml?name=Test%3A%3AUnit
  http://www.ruby-lang.org/en/raa-list.rhtml?name=RubyUnit

Curiously enough there seems to be two by Nathaniel Talbott

Any recommendations / comparisons?