Its a common feature of many sites... the "Email a friend" link that sends an email to the users' friend(s).
e.g. something like:
Hey {friends name},
I found this great site called {www.mysite.com} that lets you {verb} all your {noun}s in one place and you can share them with all your friends!
Check it out at {www.mysite.com}
Regards, {username}
I'm fairly skeptical myself and never click these links/fill out the forms but I'm presuming that I'm not like everyone else.
On the flip side I'd like to think that the current user would best know who might be interested in my site/application and thus be the ideal sales(wo)man for me.
Any stats on how successful these are would be greatly appreciated.
PS I should state that in my case I am considering this option for a Web Application vs. a Web Site (I'm not sure that makes much difference but thought I'd add that)