How to:: Site sending Notificacion Email for

Hi there...

I will like to know, If any of you guys had made this addition..

I will like the users to receive Email Notification from the site, when Someone added him/her as a Friend, and also when someone send a Comment... Can it be possible?

Thank you in advance

I'd like to know how to do this as well. Also send a notiification when a forum topic they've posted in has a new reply.