Ruby has a “pure ruby” debugger built right in. To use it:
ruby -rdebug script_name.rb
require 'debug' #somewhere in your script
If you’re on MRI, it is probably far recommended to use ruby-debug gem (which is in C, so faster), or, on 1.9, ruby-debug19 gem (same as ruby-debug gem, but written for 1.9). See a list of differences.