As a musician (amateur) and someone who loves DIY technology (amateur), the marriage between Arduino and musical instruments is an exiting area to explore. Arduino microcontrollers can be interfaced with all manner of digital and traditional musical instruments, which can be linked to MIDI sequencers such as Clavia Nordbeat or digital sound studios such as Garage Band.
Here are a few examples culled from the YouTube soup.
(Video Source: Opificio Sonico)
(Video Source: FlashUlar)