Now you may ask how is this stuff any different from say, ShareThis or AddThis. True, there is some overlap but Tell a Friend service is more about email which is still the most popular way of sharing content.
For instance, your visitors can pull a list of their friends who are currently online on Gtalk or AOL and directly send them a link of the current page. Alternatively, they may select one or more contacts from their address books inside Gmail, Yahoo! or Live Hotmail and send them a message with a link to the current page.
Another reason why some publishes may like "Tell a Friend" more is because it enables their visitors to customize the message before shooting it to their friends.
Also see: Most Useful Blog Add-ons
Update: Since some of you have express concerns over sharing of user names & passwords with a third-party, Vijay Prasanna Pullur of Pramati Technologies writes:
"The problem we address is for the visitor it is not easy to remember email addresses, so it must be made convenient to access within the context of the page. For the site/blog owner, this is a great benefit because the user is not distracted going to actual email program to fetch address and they continue to stay focused. The difference between Addthis/Sharethis is that ours is for direct communication with friends, not a social bookmarking tool which is more indirect. Of course as you said there is some overlap.
One other thing is that we dont store any email or passwords and niether the address book email addresses. We just use the login to pull the address book. As a comparison, Pramati is a 10 year old company with 300 people and large customers, so some credibility to trust our promise.
The site owner also does not get access to the visitors email address or the addresses to whom he/she sent messages using Tell-a-Friend widget. So it is safe to use this widget even on sites not well known. Whereas in email a friend form the site owner is getting the email addresses."