(Fantastic address book photo at right by moarplease on Flickr, used under a Creative Commons license.)
I have heard nothing but good things about Postable. It's an online way to collect addresses from your contacts. They simply follow your link and add their information to your book. Your addresses remain private and you can share your own address with other Postable accounts. Also, if you have your info in your Postable account and you update it, it will automatically update with all of the other Postable accounts it is linked to. You can also export your contacts as a CSV file to easily make mailing labels or what have you.
Here at Headquarters, we've decided to give it a whirl. I haven't mentioned it here before now, because, frankly, I was suspicious. It seemed too good. While we don't have the capital to do mass mailings right now, if you'd like us to have your address, you can deposit it on our Postable account. Click here to add your info to our contacts. (<- Updated the link. It should be working now.)
Do you use Postable? Tell us what you think in the comments!