According to ProgrammableWeb, there are more than 16,000 public APIs available today. That’s a lot of data that developers can access to build awesome applications. In this chapter, you’ll learn how to use the Twitter API to build a simple app that uses data from Twitter accounts. We’ll also look at how to store the app’s configuration in iOS’s Settings app. Our app is going to display a list of tweets from one or more Twitter accounts. When the app is launched, it will display a list of all available accounts to choose from. Figure 1-20 shows the app running on an iPad. Figure 1-20. The Twitterrific app running on an iPad Here are the features that our app will implement: A list of Twitter accounts in a popover view controller. The user can select one or more accounts from the list and the app will load a list of recent tweets from those accounts. After the list is loaded, the user can tap on any tweet to view the tweet’s author, text, and image. When the user adds a new account, the app will save it in iOS’s Settings app so that the user won’t have to add it again later.

There are more than 16,000 public APIs available today according to ProgrammableWeb.

Benefits of free public APIs include increased developer productivity, access to valuable data, and the ability to mash up different API features to create new value. Public APIs also offer a great way for developers to learn new programming skills and explore new technologies. Microsoft's investments in providing open and widely available public APIs, first discussed at the company's Worldwide Partner Conference in July, including the creation of the Microsoft Public License, which governs Microsoft's public APIs, and the creation of an online repository for these APIs, called MSDN Library. The availability of Microsoft's public APIs is good news for both developers and end users alike. "We think the availability of Microsoft's public APIs is a great thing," said Marc Benioff, chairman and CEO of salesforce.com, Inc. "The availability of Microsoft's public APIs helps us continue to expand our platform, which helps our partners' customers, who are also interested in other Microsoft hardware and software products, such as the new Xbox. What's more," he added, "as salesforce.

This is a lot of data that developers can access to build awesome applications.

An API is like a contract between two pieces of software. And when that contract is publicly available for anyone to access, it’s called a free public API. Developers can use free public APIs to build awesome applications without having to pay for the underlying data or infrastructure. For example, the Stripe API lets developers easily accept payments online without having to worry about setting up their own payment processing infrastructure. The benefits of using a free API are clear: you save time and money by not having to develop your own solution from scratch. But as Stripe co-founder John Collison told Business Insider, “You have to be very careful when you release a public API that it doesn’t change the rules of the game.” Now, why are public APIs so popular? For one thing, they let developers add functionality to existing applications. For another, they’re easy to get started with! Let’s look at some examples of what can be built using free public APIs. 1 Foursquare’s public API lets you add location-aware features to your app.

2. The Yelp Fusion Public API lets developers access reviews, photos, and other data to build better apps.

3. You can also use free public APIs to add things like telephony, messaging, and video chat to your app

 4. The Twitter Streaming API makes it easy for app developers to add live tweets to their app.