gem install pry pry -r ./config/environment
http://pry.github.com/
Don't miss the various extensions which are out there:
http://banisterfiend.wordpress.com/2012/02/14/the-pry-ecosys...
You can give it basic debugger stepping abilities, you can do remote sessions, you can have it automatically drop in a REPL before an exception, repair it, and carry on, it's amazing. Really shows off the power of ruby.
http://pry.github.com/