Service/Product to enable users on my Web App to invite friends (e.g. Hotmail, Yahoo Messenger, Gmail, AIM, etc)

I’m building a consumer web application and want to enable my users to invite their friends/contacts from popular email (e.g gmail, yahoo, hotmail) and social networking services to join my service. Can you suggests some products/services you’ve used in the past that enable this sort of functionality. Can you tell me what you like/don’t like about the product/service. Thx in advance for the help!