IRBEdit
IRB is short for Interactive Ruby.
You can start an Interactive Ruby session by issuing irb at the command line.
Example session
$ irb
irb(main):001:0> puts "Hello, world!"
Hello, world!
=> nil
irb(main):002:0> exit
Tips
Access the result of the last expression with _
$ 4 + 9
=> 13
$ _
=> 13
$ irb
irb(main):001:0> puts "Hello, world!"
Hello, world!
=> nil
irb(main):002:0> exit
Access the result of the last expression with _
$ 4 + 9
=> 13
$ _
=> 13
Source: http://rubyquicktips.tumblr.com/post/342527837/console-tip-retrieve-the-last-return-value-with