There are articles around for more robust ways to do this, but the following is a very simple method that I use for feedback pages.
mail( "to_friend@their_address.com", "$subject", "$msg", "From:$email" );
where
$subject is the message subject line
$msg is the message text
$email is the sender's email address
I've got this in action at my little home page:
http://www.shoutingman.com/feedback.php?subj=ShoutingMan!