It's difficult to see how you've got stuff connected up, but as the others have said you probably have an input pin whose value is undetermined ("floating"). Try the code and circuit from the button tutorial and see if that works: http://arduino.cc/en/Tutorial/Button
If you look at the page on digital pins it explains how you can use builtin resistors on the Arduino ("internal pull-ups") to do the same thing: http://arduino.cc/en/Tutorial/DigitalPins
Nice breadboard system by the way.
Andrew