An application programming interface (API) is a set of rules and specifications that software programs can follow to communicate or 'interface' with each other. As developers are well aware, there are hundreds of APIs out there for doing almost anything you could imagine online. Some are better than others, and some are definitely more useful than others. Below are forty of the most useful APIs out there. The included APIs will let you do everything from shortening a URL to displaying a book preview on your site to interacting with your Twitter account, and everything in between. Please share with us which APIs have you found most useful and feel free to recommend others that we may have missed...

The Google APIs

Google offers dozens of APIs for web designers and developers. Some are specifically related to popular Google products, like Gmail and Analytics, while others are more specialized and aren't part of public programs. All are free to use, of course. You can view all of Google's APIs and code tools on their site directory.

• Feed API - The Google Feed API lets you download any public feed (including RSS, Media RSS, and Atom) and then combine them into mashups. It simplifies the mashup process by using JavaScript rather than more complex server-side coding.
• Places API - Google Places is a large directory of local businesses and attractions all around the world. The Places API lets you access that information and display it on your website, as well as display check-ins by users.
• Geocoding API - The Geocoding API lets you convert any address into geographic coordinates, which can then be used to place markers on a map.
• Tasks API - The Tasks API offers endpoints for reading, searching, and updating Google Tasks content and metadata.
• Analytics Management API - The Analytics Management API gives improved access to your Analytics data, and lets you fine-tune your requests to just pull the information and reports you need for your application.
• Blogger Data API - The Blogger Data API lets your application create and post new blog posts, edit or delete existing posts, and search for posts based on specific criteria.
• Books API - The Google Books API lets you integrate book searches into your application, and embed book previews on your site.
• Calendar API - The Calendar API gives access to many of the standard web interface tools and operations to your web app. Public calendar events can be searched and viewed without authentication, while authenticated sessions can access private calendars, as well as edit, create, or delete those calendars.
• Moderator API - Google Moderator is a tool for collecting ideas, questions, and recommendations from any size audience. The API allows your website or application to do the same.
• Prediction API - The Prediction API helps you make smarter apps that can analyze historic data and predict future outcomes. It can be used for things like recommendation systems, spam detection, upsell opportunity analysis, and more.
• Picasa Web Albums Data API - The PWA Data API can be used to create albums and upload, retrieve, or comment on photos, among other features. It's been used for everything from powering digital photo frames to full-featured mobile clients and more.
• Static Maps API - You don't always want an interactive map on your site. Sometimes a static map is just what you need. The Static Map API lets you embed static Google Maps onto your site, including custom styled maps.
• Directions API - The Directions API lets your users get directions from one point to another using a variety of travel modes from within your site or app, and doesn't require a Google Maps API Key.
• YouTube APIs - YouTube has two APIs available: Player APIs and Data API. The Player APIs allow you to have an embedded player, or a chromeless player that you can then customize within HTML or Flash. The Data API lets your app perform a lot of the operations available on YouTube, including uploading videos and modifying user playlists.
• Webmaster Tools API - The Webmaster Tools API lets your client application use a variety of Webmaster Tools functions, including viewing sites, adding and removing sites, verifying site ownership, and submitting and deleting Sitemaps.
• Google Web Fonts API - The Web Fonts API makes it easy to add free web fonts to your website or application. Their collection of fonts grows on a continuous basis and already includes a huge variety.
• OpenSocial - OpenSocial can be used for building social applications, creating social app platforms, and sharing and accessing social data.

The Yahoo! APIs

Like Google, Yahoo! offers a number of APIs useful for developers. All are free to use and can help you integrate a variety of Yahoo!-owned web services into your app, including Flickr and Delicious.

• Answers API - The Answers API lets you access the collective knowledge contained within Yahoo! Answers. You can search Answers based on a variety of criteria (including specific user, category, and more), set your app to watch for new questions in the categories you choose, and track new answers from specific users.
• Contacts API - The Contacts API lets you access relationships in your Yahoo! address book. It reads a user's Contacts information while respecting user privacy and permission settings.
• Delicious API - The Delicious API gives read/write access to Delicious bookmarks and tags.
• Fire Eagle Developer API - The Fire Eagle API helps you create location-aware websites and applications.
• Flickr API - With the Flickr API you can view, search, and manipulate photo tags, display photos from a specific user or group, and more.
• Local API - The Local API lets you access location-based information and user-contributed content.
• Maps APIs - Yahoo! offers a number of APIs for their Maps services, including an Ajax API, a REST API, and a No Coding API.
• Meme API - Meme is a multimedia light-blogging platform. The API lets you create apps that can read, post, and repost content through Meme.
• PlaceFinder - The PlaceFinder API, similar to Google's GeoCoding API, and lets you convert a street address into geographic coordinates.

More APIs

Yahoo! and Google aren't the only ones offering powerful APIs for designers and developers. A number of social media sites and others have their own API(s), including Twitter, Facebook, Yelp, Bit.ly, and many more.

• Twitter API - Twitter has a host of developer tools surrounding their API that let you create apps that interact with virtually any of Twitter's functions.
• Facebook APIs - Facebook offers APIs for working with Credits, Ads, Chat, and more, including a couple of legacy APIs that are no longer actively supported. Also found here is the Graph API, which is the backbone of the Facebook Platform, and enables your app to read and write data to Facebook.
• Awe.sm - Awe.sm offers a number of developer APIs for integrating their social media campaign tracking tools into your app or website.
• Foursquare APIv2 - The Foursquare API not only allows you to create apps that interact with the Foursquare service, but also to use Foursquare's place-related database as a standalone service.
• Ning API - Ning offers a set of APIs for developing desktop and mobile apps, custom network features, profile apps, and data importers.
• Soundcloud API - Soundcloud's API includes tools for sharing, streaming, and customizing the Soundcloud player for your website.
• Klout API - The Klout API makes a variety of data available to developers, including Klout Scores, Network Influence, Amplification Probability, True Reach, and more.
• Social Mention API - The Social Mention API provides a stream of real-time search data from a number of social media services for integration into other applications. It's free for personal and non-commercial use.
• Opus Social Media API - The Opus Social Media API can serve as a basis for developing a social networking and digital media site or app.
• Digg API - Digg offers an API that lets you access their newsfeeds for your own sites and applications.
• Yelp API - The Yelp API lets you access business listing info, business ratings, and review excerpts from Yelp in your application or website.
• Zillow Neighborhood Information APIs - Real estate site Zillow offers APIs that give access to neighborhood information that can be integrated into other applications. (They also offer a number of other APIs, including postings, property details, home valuations, and more.)
• Tropo - The Tropo API adds Twitter, IM, voice and SMS functionality to a variety of common programming languages. Development is free, though sending messages varies in price (with Twitter and IM messages currently free).
• Bit.ly API - Bit.ly offers an API for integrating URL shortening into your app or site.

Which APIs do you use? Are there any sites you'd like to see an API from that doesn't currently offer one? Let us know in the comments!