iOS: Cobook is a smart address book that combines your default list of contacts with Facebook, Twitter and LinkedIn data. It provides you with all the information you want to have in an address book without cluttering up the interface.
Similar to our favourite iPhone Address Book, Smartr, Cobook integrates all your social networks into the app. However, Cobook is a little simpler than Smartr. Once you integrate all your accounts, simply tap a contact and their information pops up. If you want more info -- like their recent Facebook or Twitter updates -- they're right there along with all the other important contact details.
Cobook also integrates the lists from Facebook as hashtags, so you can easily search for people by keywords like "Close friends", or even by location. If your contact list is huge, you can also create your own groups.
If you use iCloud, Cobook syncs with the free Mac app, which is our favourite address book for OS X. Cobook seems simple, but it's packed with a bunch of cool features, and it's certainly worth checking out.