For less then 5 bucks, I turned mine into a way more powerful arduino.
I use the raspberry pi to send I2C commands to a microcontroller. The microcontroller lets you set pins to analog or digital, input or ouput, pwm and some other stuff, like pulse counting.
I think it's cool because you can use any language you want to program, and then keep log files and access the whole thing through ssh.
here's link to a video of it in action, just using the built-in I2C commands on the pi to dim an LED and drive a servo.