Asking your potential customers to email someone they know with your product information is an awesome marketing idea. To provide an incentive, you can promise a free gift to the one sending out the email to one of his/her friend. The friend who gets to know your product that his/her friend suggests, and the products mentioned in the email get promoted.
The all-win situation is a great way of marketing your product. Make sure your product is good enough so you won't let the sender down. Also, make sure the gift is worth the sender's effort while you minimize the work of composing the email for the sender to send out.
Use "mailto:" HTML tag to pre-compose the email, set the b.c.c. to yourself so you also get a copy every time the email gets sent out. Then, you process one email a time to deliver the gift to the sender.
Here's an real case example I just received using this approach:
I'm thinking of applying this approach my paid-customers who bought the information product (a 12 months membership). I'll encourage them to email another person to spread the words in exchange for a free bonus product.