Viral Marketing: Which open-source program is best to use to implement an email inviter - where one can invite their friends on a service?

Depending on your needs and the barrier to entry to your service (requirements for account creation, incentives for inviting people, etc...), you can actually re-purpose add-this to handle your referral program. When I was at Bloomspot, we were able to get up a simple 'refer a friend' page with http://AddThis.com and minimal engineering support. It's nice because you can customize your facebook update, email, contact back integration, twitter, and custom link referrals quite easily. There are limitations to the service and it may not be as customizable as you like, but it works. https://www.bloomspot.com/referrals/invite/first-purchase/#

Sounds like you're looking for a referral program that includes email capture... Friendbuy is http://www.friendbuy.com that gives sophisticated marketers the ability to quickly launch, track  and  analyze social referral performance. Once live, improving results  with  A/B testing is just a few clicks away. If   you have a highly trafficked site and large customer base, you’ll want   to take advantage of advanced features, mirroring the way you oversee   all of your customer acquisition efforts: Optimize your performance through A/B testing Identify customers who've self-referred via fraud detection Automate using API/Webhooks (also great for scripting business rules such as ‘refer 3 get 1 free.’) So check out Friendbuy - you can https://retailer.friendbuy.com/signup/account_info

http://stescodes.com/grabcontacts.aspx also looks good. Although it's called "Facebook" contact importer, it actually works with email services.