Test Coverage Tool for TDD?

Hi,

I'm learning TDD without using Shoulda or RSpec. When I played with
RSpec, there was a tool to show test coverage visually.

Is there any similar function built-in to Rails Test::Unit?
Or do I need to add it on my own?
What's the recommended tool?

I tried `rake -T test`, but didn't find anything about coverage.

-Jon

Yup, it's called rcov: http://eigenclass.org/hiki.rb?rcov

--Jeremy