This is a recent project to mod a broken Guitar Hero World Tour drum controller from a yard sale into a real electronic drum kit.  The nitty gritty details are in the blog but basically I opened the case and attached all the piezo electric drum triggers in the kit to an Arduino, and then used the Arduino to trigger MIDI notes that played sounds through a PC.  Overall, it turned out well.  It was my first time using an Arduino and I completely love the little thing now.  Here's a video of my son playing the kit after we finished it.  I had a great time making it and I'm now looking for my next Arduino project to undertake.