Contact list based referral system, preferably using PHP/open source but alternatively commercial solutions
Im looking for an application (preferably open source, free, php) that can easily be included into my existing site, where my clients can enter their hotmail/gmail/yahoo (++) account credentials and it will automatically gather all the associated email adresses and eventually let them send invitations etc. to the selected people.
(Like facebook, and also widely used in msn blockchecker tools etc.)
I have an intermediate understanding of PHP and MYSQL, so i would appreciate a solution based on such a technology, but any solution, even commercial and payment required is greatly appreciated!