If you are looking for a tell-a-friend script with captcha (Completely Automated Public Turing test to tell Computers and Humans Apart), you should take a look at this post at Dan Moran´s blog. He has just built it himself.
I have tested it and it seems great. With the captcha it´s a lot more secure than other tell-a-friend scripts, and this way you will almost know for sure that only humans will be using it. In a couple of days he will be launching his new website at www.formdefender.com. I am not sure if the script will be free or not.