Gone are the days when you kept all your contacts in one place. Nowadays they're probably spread out across not only your address book, but also your social networks.
Cobook for iOS puts your local and social-network contacts under one roof. In other words, it unifies the data from your address book and your Facebook, LinkedIn, and/or Twitter accounts.
It's a simple matter to set all this up. Just give Cobook permission to access your contacts (it doesn't upload anything, a nice change from some other address-book replacements), then do the one-time permission drill for whatever networks you to plan to include.
With that done, you can view all your contacts in one list, or filter them by source. Cobook also imports your Facebook lists so you can view those contacts, and it has three "smart" lists: no email, no phone, and no photo. In other words, if you want to see only those contacts that have, say, no attached phone number, you can.
Another big perk: you can create contact groups and merge contacts, the latter an especially nice option if you find duplicates. What you can't do, however, is exclude Twitter contacts that aren't actually contacts, but rather just people you follow for fun. (I'm not actually how Cobook would allow you to make that distinction, but I found it weird to see celebrities like Conan O'Brien alongside my business contacts.)
When you view any given contact, you see not only their name, phone, email, and other common details, but also their latest tweets, updates, and the like (if they're part of your social networks). And Cobook will turn any contact into a shareable "card" you can email, including as much or as little information as you want.
The app does have a few flaws. For example, there's no support for FaceTime, so you can't easily place a video call to your iPhone contacts; you still need the stock Contacts app for that. What's more, I couldn't get Cobook to connect properly with my LinkedIn account. It repeatedly displayed a "service error" message. Even after removing and reconnecting that source, it wouldn't work. Gotta ping tech support on that one.
Even so, I'm planning to give Cobook a permanent home on my iPhone, as it easily unifies all my disparate contacts, and looks lovely while doing it. Plus, you can't beat the price: Cobook is free.
Have you found an address-book alternative you like better? Tell me about it in the comments.