Well, today's your lucky day: the Arduino IDE comes with a pseudo-random generator: random.
Pieter
P.S: If you don't connect anything to an analog input, it will start to oscillate, producing all kinds of garbage values, depending on how close you put your hands to the pin. If you wanted to, you could measure a floating analog input to get a "random" seed for the random() function. But I think it will be too periodic to use as an actual pseudo-random value, and some values are much more probable than others.