Tell a Friend Viral Forms
You can have this either as a form within your site, or you can use it this way:
The great thing about this particular script compared to some of the others, if that you can write your own sales pitch in the script that the person sending doesnt see, try it out, send it to yourself and see what I mean.
Also this is a self hosted form, which in laymans terms means no ad's complete privacy, no pop ups unless you choose to go with a button, and thats running in java that a user activates so it gets past most of the pop up blockers.