IDE which allows STEP-THROUGH? - Does one exist?

Greg,

I've been using the console for this. (ruby scripts/console).
Typically I only need to test individual functions and it only takes a
couple seconds to setup the objects. Then I copy individual lines of
the function over. It may seem cumbersome, but I've actually found it
to be quite quick.

Christer - did you compare with Ruby In Steel (as suggested by Huw)?