Developer: Facebook, Inc.
Price: free Download on the App Store
If, like me you are a heavy Facebook user, the Facebook app for the iPhone is a must have. Of course, it's completely free. And it's so good, you may never go back to the Facebook website again.
The Facebook iPhone app is broken into 5 tabs: home, profile, friends, chat and inbox. On home, you see the Facebook feed of all your friends. Since many people use twitter to update their Facebook status, it almost makes twitter irrelevant. The profile page allows you to browse your Facebook photo library, visit your wall, or update your personal info.
The friends tab is probably the most interesting. It gives you one touch access to all of your Facebook friends. Given that it is a lot easier to friend somebody on the Facebook site, this in some sense becomes your "uber-address book". Similarily, the "chat" tab allows you to ping anybody who is online and start an instant message conversation with them: brilliant.
Finally, the inbox gives you immediate access to your Facebook mails. And it does it without asking you to login every time.