I just learned about an educational kit that includes an Arduino-compatible board plus enough components to keep you busy for quite a while, including sound and RF shields. The board accepts standard Arduino shields, but has multiple takeoff points for 5V and GND plus headers for I2C, SPI, and others. It can also sink higher pin currents. You can see more info at:
http://arduino-info.wikispaces.com/YourDuinoEngStarter
I have no financial interest in this product...just thought it could be useful.