Previously I've used OpenInviter to import user contacts, but it tends to break and remains broken for months. I've switched to CloudSponge and now everyone's happy.

- Works with the 4 major webmail services plus Outlook and Mac Address Book

- Very simple to integrate

- Looks good out of the box, but can be customized

- You can choose to use their API directly for full control

Pricing is $25 per month (first month is free).

Disclaimer: I'm not associated with CloudSponge in any way, just a fan.