Instructables staff member Randy Sarafan has documented one of the largest Arduino-based projects we have seen for some time, his Robot Drum Kit. It isn't a bunch of robots playing the drums, instead each drum is played by an Arduino with two drumsticks controlled by car door lock motors and motor shields. Making the system play is a sequencing program for MAX MSP which controls the Arduinos via USB. The entire project is simply amazing and worth a look here:
If you're interested in more advanced Arduino projects and usage - get yourself a copy of "Practical Arduino" by Jonathan Oxer and Hugh Blemings:
Create your own Arduino-based designs, gain an in-depth knowledge of the architecture of Arduino, and learn the easy-to-use Arduino language all in the context of practical projects that you can build yourself at home. Get hands-on experience using a variety of projects and recipes for everything from home automation to test equipment. For more information and to order, click here.