Old metallophone (toy) rejuvenated by Arduino, some electromagnets and printed parts :
https://vimeo.com/46571145
Detail :
https://vimeo.com/47593529
 
Can be played with mouse, keyboard and midi files,
samples included in software pack:
Georges Brassens - Ballade des dames du temps jadis
Agnes Obel - Riverside
Yann Tiersen - Comptine d'un autre été
Julian Nott - Wallace and Gromit
Michael Giacchino - Married Life (Pixar Up soundtrack)

Flash (AS2) on the software side:
http://agendaz.net/chaaawa/xylobot/
 
Thanks to midicsv:
http://www.fourmilab.ch/webtools/midicsv/
 
Not the first xylobot in the world, but maybe the first in the Thingiverse !
Next step, same thing with homemade solenoids :
http://www.thingiverse.com/thing:6049
http://www.thingiverse.com/thing:5950
... and wooden bars
to make it sound more "xylo"

Print as many holders as you have bars.
See pictures for assembly.
ULN2003 on the breadboard (see assembly diagram)
Upload .pde file to your Arduino board.
Install and configure a server proxy (for communication between Arduino and Flash)
Serproxy and Tinkerproxy included in software pack for OS 10.5 and 10.6. Equivalent available for all platform on http://arduino.cc
In your proxy server config file find the line containing "serial_device1" and give the identifier of your board
Launch server proxy.
Open index.html with your browser, or Arduino_xylobot.swf with standalone Flash Player (add trusted location when prompted).
Can be used as:

• ringtone
• doorbell
• alarm clock
• memory game
• educational toy
• ...
  A last one, Riverside (Agnes Obel) :
  https://vimeo.com/47593782