The iPhone App Store from Apple has hundreds of thousands of applications of various types available for download. Download an ebook reader and take your favorite novels with you on the road, or download a game to keep yourself occupied during a long car ride. If you want to share your favorite iPhone applications between friends, you have a few different ways that you can do this.
Tell your friends the name of the application you want them to download. They will then be able to click on the "App Store" icon on their iPhone and download that specific application using their own data plan.
Download the Bump application. To do this, tap the "App Store" icon and search for "Bump." This is a free application that is designed to allow two iPhones to share information, including applications. Note that for this to work both iPhones must have the Bump application and must be connected to the same Wi-Fi network.
Authorize your friend's computer. If you log in to your iTunes Store account on a friend's computer, you can then download all of the applications you've downloaded on your iPhone (including ones that you've paid for) to their machine, which they will then be able to use on their iPhone. Note that each iTunes account can authorize only up to five computers in a lifetime.