Noteboard is a project mainly written in Ruby, it's free.
It's your keyboard, only with music.
Noteboard allows you to play music using your computer keyboard. It requires jruby.
You need to have Garageband running at the moment, pick a soft instrument. Then, run
jruby noteboard.rb
You should then be able to play notes on your keyboard. Enjoy!
Noteboard was created by Toby Ho.