My personal favourite:

    gem install pry
    pry -r ./config/environment

(or add pry-rails to your Gemfile)

http://pry.github.com/

Pry is indeed amazing.

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.

Don't forget Ripl, either: https://github.com/cldwalker/ripl

it would be nice to have a community site where people can share some tricks in any programming languages/tools.

Similar site I have been waiting for since Feb: http://rubyreloaded.com/trickshots/