While the iPhone tries to provide a fulfilling experience out of the box, it uses its App Store to customize it for you. Below are a few apps that have proved to be incredibly useful for most users:
Facebook for iPhone This app allows you to stay connected with others on Facebook, just as you would from your desktop computer
Wikipanion Allows you to make full use of Wikipedia from the convenience of your iPhone. 
Google While you can search from Safari, it is easier to use this app to get results.
Google Voice for iPhone If you know what GoogleVoice is and why it’s so great and have an iPhone, this is a must have.
ESPN ScoreCenter Allows you to check the scores on all sports that ESPN provides scores on.
Yelp This app is user-friendly portable version of the online restaurant and bar review site that is rated by the users.
Pandora Allows you to stream music that is similar to your search string (e.g. if you search for Dave Matthews Band, songs by DMB and songs from other artists that are similar will appear in the streaming song list.)
Shazam Allows you to let the phone “listen” to music playing and will identify the artist and song and provide a link to iTunes for you to download the song or album.
Fring This is an instant messaging client that can be used with AIM, MSN, ICQ, Yahoo, and others.
WeatherBug This app provides more information that the Weather app that comes with the iPhone.
WSJ (Wall Street Journal) If you are a fan of the Wall Street Journal, this app will provide those stories in a portable, free format.
Flixster This app is great for looking up movie times, theater info, buying tickets, etc.
Google Earth While the Google Maps app included with the iPhone is great (“Maps”), Google Earth provide a more dynamic interface for exploring locations and viewing them. Maps is still better for navigation.
Evernote Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at any time, from anywhere. The bonus? It’s free. For a great summary of how it can work for you, take a look on Lifehacker’s summary.
SiriusXM Online If you have a SiriusXM Online account, you can listen to any of the XM or Sirius stations through your iPhone.
CoolFacts If you a fan of reading random facts, this app is for you. Disclaimer: Not all the facts have been verified.
epicurious If you are ever in search of a recipe, this is a great place to go. Best of all, it’s always with you as long as you have your phone. At the grocery store and need some ideas or need to figure out if you have all the ingredients? This is the app for you.
NPR News If you are an NPR junkie, this app is for you. It allows you to stream your favorite stations or download podcasts for free.
Gizmodo’s List Essential iPhone Apps (constantly updated) Gizmodo does a great job of putting together the most useful apps out there, including games, connectivity programs, media sourcing programs, and information gathering programs.
100 (Really Beautiful) iPhone Wallpapers This site does a great job collecting an array of different wallpapers which are formatted for the iPhone. If you want a new look for your device, take a look here.
Classic iPhone Face Wallpapers If you want a touch of irony and humor for your iPhone, take a look at these backgrounds.
