Stefani,
I used to have one on my site. I used the one at .
It's just a simple one. Someone clicks on your "Recommend Us" link or image, they are sent to a form (you can customize this to fit your site design, you can also choose the number of target recipients), they fill in the form with their friends' name and email and after submitting the form they return to the page they were from.
Is this what you were looking for?
Warm regards,
Bina.