It's increasingly rare that you ever need a friend's physical address anymore, but there are occasions where you need to send out a physical letter. If you don't feel like emailing, calling or posting a Facebook update to track down addresses, Postable is a dead simple and secure address book where your friends can enter their own info.
After you set up your account you receive a link with your address book. Send that link out to your friends and they can add their information. Your friends can only add their information. They can't see other entries in your address book and the site is secured. Once you get everyone's address, you can export your address book to Apple Address Book, Microsoft Outlook, Gmail, Excel and even print labels.