> A more pervasive example is the “Sent from my iPhone/iPad” signature at the bottom of every email you send from your iPhone or iPad. Yes, even Apple is using viral tactics (actually, I believe BlackBerry started that with “Sent using BlackBerry”).

As noted in "Founders at Work", Hotmail used this tactic with a lot of success. I don't know if they were the first to try it, but they were definitely the first widespread example.