What a Web you work on! Technology from a wide variety of companies and industries is freely available for you, a designer/developer, to capture and leverage for your own or your clients benefit.
If you're a company considering publishing an API, taking a look at the successes of others might encourage you to complete that project. iContact for example just announced the winners of its API Challenge, a content for partners and customers that asked them to build an integration between third party apps and iContact. The result, in addition to greater exposure for their service, was a lot of new mashup technology that can be used by the greater Web community (not just iContact clients).
Let's review a few other great API's for your next website or application now:
NameCheap Website Management Tools: The API for NameCheap, which offers a domain name registration, enables developers to build web or desktop apps that integrate with their NameCheap account. Users can access account data from NameCheap.com and offers features like domain check, registrations and renewals from within applications.
Hexillion Whois Query Service: Another from the domain name category, the Hexillion Whois API provides a programmable interface to the Whois system, automatically querying the correct Whois server, and breaking the Whois record details out into separate fields. The API, which returns XML, is built for higher volume use.
CheddarGetter Subscription Payment and Management Service: The API from online billing and subscription management system CheddarGetter makes it possible to accept and track payments and integrate the service into their website or app.
Spreedly Subscription Payment Service: The API from Spreedly gives web apps a way to handle subscription-based services. The service works through many payment gateways, including PayPal and SagePay, to complete transactions.
Zendesk Help Desk Service: Help desk system Zendesk's API allows developers to hook into Zendesk and connect it to third-party applications. Whether you're writing a plugin for an application or planning on hooking some internal application into Zendesk, the API can do it for you.
Amplify Content Analysis Service: Amplify is a web service that brings "human understanding" to content. Using its patented Natural Language Processing technology, Amplify reads and understands words used in text, identifying the significant topics, brands, people, perspectives, emotions, actions and timescales and presents the findings in an actionable XML structure.
Klout Social Metrics Service API: Klout allows for impact tracking of opinions, links and recommendations across a social graph, by collecting data about the content created and how people interact with that content (as well as the size and composition of the network).
Proximic Content Advertising Service API for Publishers: The Proximic API allows publishers to display ad units and track the performances of their pages against days, URL referrers, ad unit formats, or ad groups. The Proximic web service also gives software developers direct access to the basic and extended functions needed for contextual advertisement publishing and reporting purposes.