ruby -rpry some_script.rb - the r means require. If there's a
binding.pry there, you'll be on it.
Pry gives you a graphical look at your program in progress, lets you
cd among objects,
ls to see available variables, etc. You can't step, though; just explore a snapshot at that moment.
Must install debugger gem for your version of Ruby (1.9 is 'gem install debugger'). Doesn't play well with Spork.