Mike Filsaime has a pretty cool tell-a-friend "viral" script about to go on sale. I have a copy of it, and I'm going to be testing it shortly.
Just like any opt in or lead generation device, you need to give them an incentive to spread the word. And it should be something with a high perceived value.
But once they tell 3 friends, for example, the exponential part begins. Because those 3 friends tell 3 friends, who tell 3 friends, and so on (obviously not everyone will participate).
I'll let you know how it pans out for me.
EDIT: I just saw this free one linked on the Warrior Forum:
:: Enough Is Enough! ::
John