Get user's Gmail/Hotmail/Yahoo contacts for invites

I'd like to allow my users to invite their friends by pulling in their gmail/hotmail/yahoo contacts from their email account. We already have a totally functional site in PHP/MySQL. I've programmed a majority of this myself and I'm aware of some free scripts out there that can do this - but would prefer to have someone else implement it - particularly if they've done this before (since I have not)