Mac: The default address book in Mac OS X isn't bad, but it could do a lot more. Cobook is an Address Book app for Mac that sits in your menu bar, offers quick mouseless navigation, and lets you add a lot more to your contact cards, including Facebook, Twitter and LinkedIn accounts.
Cobook is a much easier way to manage your contacts than what comes bundled with Mac OS X. It has one simple text box at the top of the drop-down window, in which you can type a contact's name to quickly bring them up. You can then add anything to the card by typing it in that same box -- it will recognise that you're typing an email address, phone number or other piece of information and add it to the right slot.
You can also add social networking profiles and view them right from the app, not to mention add pictures and other information automatically from those sources. And, best of all, it stays in sync with Address Book and iCloud, so you don't need to do any work to keep that information synced to all your devices. Check out the video above to see it in action.
Cobook is currently in free beta, and is available for Mac OS X only.