I do not think there is any thrill that can
go through the human heart like that felt by the inventor
as he sees some creation of the brain unfolding to success . . .
Such emotions make a man forget food, sleep, friends, love, everything.

Nikola Tesla

Free Rice

Check out Free Rice.
It's a most excellent example of viral, extremely rapid growth.
A spectacularly simple, win-win, business model.